Change the tmp path for the vagrant_projects directory

- Change the tmp path for the vagrant_projects directory to be under pro...jects root directory at '.rspec_system'
- Add environment variable, RSPEC_SYSTEM_TMP,  to override the TMP path.
- Issue #14

Example output of new default

```
$ bundle exec rake spec:system
<snip>
=================================================================

Commencing rspec-system tests
Total Test Count: 4

Starting nodes

Setname is: centos-64-x64
Configuration is: {"nodes"=>{"main.vm"=>{"prefab"=>"centos-64-x64"}}}
Virtual Environment type is: vagrant
Default node is: main.vm
Destroy node is: true

INFO: [Vagrant#setup] Begin setting up vagrant
INFO: [Vagrant#create_vagrantfile] Creating vagrant file here: /Users/treydock/puppet/modules/dh/.rspec_system/vagrant_projects/centos-64-x64
DEBUG: Filling in content for main.vm
DEBUG: [Vagrant#create_vagrantfile] Finished creating vagrant file
```

Example of output with override used

```
$ RSPEC_SYSTEM_TMP=/tmp rake spec:system
<snip>
=================================================================

Commencing rspec-system tests
Total Test Count: 4

Starting nodes

Setname is: centos-64-x64
Configuration is: {"nodes"=>{"main.vm"=>{"prefab"=>"centos-64-x64"}}}
Virtual Environment type is: vagrant
Default node is: main.vm
Destroy node is: true

INFO: [Vagrant#setup] Begin setting up vagrant
INFO: [Vagrant#create_vagrantfile] Creating vagrant file here: /tmp/vagrant_projects/centos-64-x64
DEBUG: Filling in content for main.vm
DEBUG: [Vagrant#create_vagrantfile] Finished creating vagrant file
```