Fix zuul installation

The following items were missing from an automated zuul install:

 * A pre-populated known hosts file
 * The git user/email for creating commits (.gitconfig existed
   on zuul.o.o, but switch to using zuul's config file instead).

Also, make sure that the mergers specify the zuul server as the
gearman server address rather than localhost.

Change-Id: I47a473f60c4b5b2daaa910aa61cdf6e0c6fe1528