Add a way for the distro config to force dependencies

Instead of just having multipip force dependencies allow
for a master override to be provided via a distro config
key 'forced_pips' that will override multipip's output and
any decision/requirement from it or from requirement files.

Change-Id: Ie189368d44edb6c5df24b8dfd96ec17897dc86df
Merge "Add a way for the distro config to force dependencies"