Add environment resource attributer to scm resources

Allow run git command with environment variables.

For example,  A remote repository which provides by https using self-signed cert.

```
git '/var/tmp/example' do
  repository 'https://unsecure.example.com/example.git'
  reference 'master'
  action :sync
  environment "GIT_SSL_NO_VERIFY" => "1"
end
```

Result of unit test below.

```
$ bundle exec rspec spec/unit/provider/git_spec.rb spec/unit/resource/git_spec.rb 
DL is deprecated, please use Fiddle
Run options:
  include {:focus=>true}
  exclude {:provider=>#<Proc:./spec/spec_helper.rb:136>, :arch=>#<Proc:./spec/spec_helper.rb:130>, :requires_root_or_running_windows=>true, :requires_root=>true, :ruby_20_only=>true, :ruby_19_only=>true, :ruby_18_only=>true, :selinux_only=>true, :system_windows_service_gem_only=>true, :solaris_only=>true, :windows_domain_joined_only=>true, :windows32_only=>true, :windows64_only=>true, :win2k3_only=>true, :windows_only=>true, :volatile=>true, :external=>true}

All examples were filtered out; ignoring {:focus=>true}

Chef::Provider::Git
  creates a current_resource with the currently deployed revision when a clone exists in the destination dir
  keeps the node and resource passed to it on initialize
  responds to :revision_slug as an alias for target_revision
  runs a clone command with escaped destination
  compiles a clone command using --depth for shallow cloning
  compiles a clone command with a remote other than ``origin''
  runs a checkout command with default options
  runs an enable_submodule command
  does nothing for enable_submodules if resource.enable_submodules #=> false
  runs a sync command with default options
  runs a sync command with the user and group specified in the resource
  configures remote tracking branches when remote is ``origin''
  configures remote tracking branches when remote is not ``origin''
  raises an error if the git clone command would fail because the enclosing directory doesn't exist
  does a checkout by cloning the repo and then enabling submodules
  does not call checkout if enable_checkout is false
  does a checkout by cloning the repo and then enabling submodules when the directory entries are listed as %w{.. .}
  should not checkout if the destination exists or is a non empty directory
  syncs the code by updating the source when the repo has already been checked out
  marks the resource as updated when the repo is updated and gets a new version
  does not fetch any updates if the remote revision matches the current revision
  clones the repo instead of fetching it if the deploy directory doesn't exist
  clones the repo instead of fetching updates if the deploy directory is empty
  does an export by cloning the repo then removing the .git directory
  determining the revision of the currently deployed checkout
    sets the current revision to nil if the deploy dir does not exist
    determines the current revision when there is one
    gives the current revision as nil when there is no current revision
  resolving revisions to a SHA
    returns resource.revision as is if revision is already a full SHA
    converts resource.revision from a tag to a SHA
    converts resource.revision from an annotated tag to the tagged SHA (not SHA of tag)
    raises an invalid remote reference error if you try to deploy from ``origin'' and assertions are run
    raises an unresolvable git reference error if the revision can't be resolved to any revision and assertions are run
    does not raise an error if the revision can't be resolved when assertions are not run
    does not raise an error when the revision is valid and assertions are run.
    gives the latest HEAD revision SHA if nothing is specified
  with an ssh wrapper
    without a timeout set
      clones a repo with default git options
    with a timeout set
      clones a repo with amended git options
  configuring remote tracking branches
    checks if a remote with this name already exists
    runs the config with the user and group specified in the resource
    when a remote with a given name hasn't been configured yet
      adds a new remote
    when a remote with a given name has already been configured
      updates remote url when the url is different
      doesn't update remote url when the url is the same
      resets remote url when it has multiple values
  calling add_remotes
    adds a new remote for each entry in additional remotes hash
  calling multiple_remotes?
    when check remote command returns with status 2
      returns true
    when check remote command returns with status 0
      returns false
    when check remote command returns with status 0
      returns false
  calling remote_matches?
    when output of the check remote command matches the repository url
      returns true
    when output of the check remote command doesn't match the repository url
      returns false

Chef::Resource::Git
  is a kind of Scm Resource
  uses the git provider
  uses aliases revision as branch
  aliases revision as reference

Finished in 0.04756 seconds
53 examples, 0 failures
```
I think deleting HOME that way is fine.

But, you need to be careful here with passing run_opts[:environment]. Note that in the previous code we weren't setting it at all if it is empty, but you're unintentionally setting it to an empty hash if both environment and user are not specified as attributes.

Passing "run_opts[:environment] = nil" has a special meaning. I think in this case passing an empty hash wouldn't trigger that special meaning, but I think it would be best to keep the current behavior of not setting run_opts[:environment] at all if neither environment or user are set.

https://github.com/opscode/mixlib-shellout/blob/master/lib/mixlib/shellout.rb#L298