Update docs to reflect everywhere wskdeploy does interpolation on manifest values

[mrutkows]

It became apparent to me that we have not kept the docs current as to what values in the manifest now support interpolation (and more could!).

This involves going through the manifest parser: https://github.com/mrutkows/openwhisk-wskdeploy/blob/master/parsers/manifest_parser.go

and look for all places where a value is extracted via the "InterpolateStringWithEnvVar()" function and document it here:
https://github.com/apache/openwhisk-wskdeploy/blob/master/docs/wskdeploy_interpolation.md

[morrme]

I'm not sure how to properly document this in the specified file, but if it helps, the occurences are :

Line 195
Line 248
Line 324
Line 641