(GH-172) Enable use of class-based DSC Resources by munging PSModulePath

[michaeltlombardi]

Prior to this PR, it was not possible to use the DSC Base Provider to invoke class-based DSC Resources. DSC 1.1 (the version of DSC used by Windows PowerShell 5.1) cannot call Invoke-DscResource for class- based resources by module path, only by module name; further more, the module containing the DSC Resource must be available in the system level PSModulePath environment variable; munging the process level PSModulePath does not work.

This PR updates the DSC Base Provider to:

1. Read the dscmeta_resource_implementation value from a the Puppet Resource API type definition of a resource and inject it into the invocable resource definition
2. Add the munge_psmodulepath method to return the PowerShell code to modify the system-level PSModulePath if the invocable resource is is implemented as a PowerShell class; if the resource is any other implementation it instead returns nil
3. Modify the invoke_params method to set the ModuleName key to the PowerShell module's name instead of the path to the module's manifest if the resource is implemented as a PowerShell class
4. Inject the PowerShell code from munge_psmodulepath into the output from the ps_script_content method to ensure it will be called when invoking the resource
5. Update the try/catch in the postscript code file to have a finally block which resets the PSModulePath if it was munged earlier in the invocation script, regardless of whether or not the invocation errors

TODO:

• Add acceptance tests for class-based resources
• Add unit tests for munge_psmodulepath
• All existing tests pass (update if needed)