Recognize "lp" as meaning Launchpad bug in gerrit strings

Gerrit changes 312 and 320 used "LP" as an abbreviation for
Launchpad bug.  When followed by a number, no link was
created to the correspnding bug number.

This changes treats "LP" the same as "bug" when creating links
to launchpad.net.

Change-Id: I1ef1676dde423252e25bbe69a53f978b44da9810