Run nova post jobs on trusty nodes

Nova has libvirt-python in its test-requirements.txt which means we need
to run all master py27 nova jobs on trusty. Update the node selection
regex in zuul layout to continue to run ^gate-nova.* jobs on trusty as
well as ^nova-.* jobs on trusty.

Change-Id: I8613c1da59630ce693fd11b19ab3fbe095a60c89
Merge "Run nova post jobs on trusty nodes"