It appears that there can only be one skip-if and you have to figure
out how to encode all of the conditions you want into one skip-if.
Luckily, this is sufficient for trove's usage; skip the job on two
projects if the branches are not stable/liberty/mitaka or newton.
For example, see http://logs.openstack.org/93/382493/4/check/. This
was a change on master and both the legacy and non-legacy jobs got
run. They would pass now because the contents of the two repos have
not diverged; that's not the point, they shouldn't both be run :)
Change-Id: I8609d16b3442374ec1c44ba5b1ec247342b3fe7c
f3b2da9643