Allow configurable mappings to different LP projects

This allows anyone to use DocImpact tags, with non-core projects
having their bugs mapped to their own LP projects, instead of
openstack-manuals. This is required because there is at least
one stackforge project that would like to use DocImpact as part of
their development process, but they can't assume support from the
OpenStack docs team at this time.

Change-Id: I6c9a9f3f42b3a9eff33ca0522b9f23f058131433
Merge "Allow configurable mappings to different LP projects"