Limit python26 jobs to icehouse/juno for server projects

For server projects in the openstack namespace, limit the python26 jobs
to icehouse/juno branches only.

Change-Id: Ib495467e00e4b6abf2b3c2b9445b51652ba25167
Co-Authored-By: Michael Krotscheck<krotscheck@gmail.com>
Merge "Limit python26 jobs to icehouse/juno for server projects"