Allow conditional inclusion/exclusion of Galaxy tools and/or tool 'provides' synonyms based on fabric environment variables. Add 'if: <variable>' to the yaml config below the tool or the provides will cause the tool configuration to only be installed if that fabric variable is True (according to the _read_boolean function - i.e. if the variable is "true", True, 'YES', etc...). Likewise if an 'unless: <variable>' appears the tool will be installed unless the fabric variable is True.
Correctly ignore additional custom packages when not specified. Thanks to @roryk