Added support for conda metapackages

This merge adds support for building conda metapackages from the same YAML definitions files as is currently done for RPM/Deb.

Summary:

• generate.rb has been replaced by generate.py (should give identical results for rhel/deb)
• added conda package entries in appropriate metapackage YAML files
• added support for a skip key in the YAML files, allowing a platform to be ignored for a given metpackage - this is done in a few places where dependencies are not available yet in conda-forge