[FEATURE] Simplify inheritance of option vals for project create variable group cmd

[dennisseah]

closes microsoft/bedrock#1311

added inherit properties to command decorator json file and use
populateInheritValueFromConfig and validateForRequiredValues functions to inherit values from config.xml and check that all mandatory options have values respectively. hence removing code of performing the above tasks in create-variable-group.ts file

[dennisseah]

validations.sh results

pipeline: fabrikam-hld-to-fabrikam-manifests-420:
pipeline_status this iteration --> "completed"
Successful build for pipeline: fabrikam-hld-to-fabrikam-manifests-420!
info:    Generating config.yaml from yaml file input.
info:    config.yaml was generated and located at /Users/veseah/.spk/config.yaml.
info:    Successfully initialized the spk tool!
Start Time           Service      Deployment   Commit  Src to ACR Image Tag                                             Result ACR to HLD Env    Hld Commit Result HLD to Manifest Result
4/6/2020, 4:42:36 PM fabrikam2019 8af24cbec99a 7ad618b 699        fabrikam2019-fabrikam.acme.frontend:master-20200406.1 ✓      699        MASTER 20ab109    ✓      700             ✓     
1 deployment(s) were returned by spk deployment get
Verified that srcPipeline:699 acrPipelineId:699 hldPipelineId:700 pullrequest:"361" are all linked together correctly.
Successfully reached the end of spk deployment get tests.