Add configurable defaults for chef-solo & chef-client paths.

This commit adds 3 new tunables: 1 for all Chef provisioners, 1 for
`chef_solo`, and 1 for `chef_zero` and builds on @sethvargo's previous
commit.

**:chef_omnibus_root** which defaults to `"/opt/chef"` is the location in
which Kitchen will check when determining if a Chef Omnibus package is
installed. Modifying this attribute will allow users to use alternative
Omnibus packages and is most likely always reasonable with its default
value.

**:chef_solo_path** is used with the `chef_solo` provisioner, and is the
full path on the remote instance to the `chef-solo` binary, and by
default is calculated using the `:chef_omnibus_root` attribute as its
base. For example, if the Omnibus root is `"/opt/chef"`, then the
`:chef_solo_path` value will be calculated as
`"/opt/chef/bin/chef-solo"`. If the `chef-solo` command is located
elsewhere, this value can be set in a provisioner block to override the
default. For example, setting this attribute to `"chef-solo"` will
invoke the command without an absolute path and rely on `$PATH` lookup.

**:chef_client_path** is used with the `chef_zero` provisioner, and is
the full path on the remote instance to the `chef-client` binary, and
by default is calculated using the `:chef_omnibus_root` attribute as its
base. For example, if the Omnibus root is `"/opt/chef"`, then the
`:chef_client_path` value will be calculated as
`"/opt/chef/bin/chef-client"`. If the `chef-client` command is located
elsewhere, this value can be set in a provisioner block to override the
default. For example, setting this attribute to `"chef-client"` will
invoke the command without an absolute path an rely on `$PATH` lookup.

Here is a concrete example of overriding `:chef_solo_path` and
`chef_client_path` in a `.kitchen.yml` file:

    ---
    driver:
      name: vagrant

    platforms:
      - name: ubuntu-14.04-solo
        provisioner:
          name: chef_solo
          chef_solo_path: /mnt/alt/bin/chef-solo
      - name: ubuntu-14.04-client
        provisioner:
          name: chef_zero
          chef_client_path: /bin/chef-client

    suites:
      - name: default

**Warning:** previous logic used a non-absolute path to invoke both
`chef-solo` and `chef-zero` which means that this commit has the
potential to cause some Kitchen runs to fail, assuming that the Chef
being invoked before was not actually `/opt/chef/bin/chef-solo` or
`/opt/chef/bin/chef-client`. In this case, users can override the new
computed default with a value of `"chef-solo"` or `"chef-client"` to
exactly preserve previous behavior.