Filtering on validate

[lennyphan]

To support generating configuration overrides with model filtering , for Model-In-Image (MII) domain home source type, the following changes are needed in WDT:

1. Add the ability to apply filters for The Validate Model Tool (validateModel),
2. Persist model after filtering has been applied.

For MII, validateModel tool is used to generate merged models and filtering is needed to apply configuration overrides.

A couple unit tests created to verify that model filter(s) are invoked (for validateModel tool) and to verify that filter changes are persisted, if configured.

[jshum2479]

LGTM

[rakillen]

the model should still have variables unresolved before going into validate_in_standalone_mode, so that any missing variables can be logged there.