Skip Nova exercises if Nova is not enabled

This allows for ./exercises.sh to complete sucessfully when nova is not
enabled / installed.

Change-Id: If969e14f5106c15007146e8fad1da27d131828c8
Merge "Skip Nova exercises if Nova is not enabled"
Merge commit 'refs/changes/97/61397/2' of git://gold.eng.hq.xensource.com/git/github/openstack-dev/devstack into tmpbranch
Merge commit 'refs/changes/22/61422/2' of git://gold.eng.hq.xensource.com/git/github/openstack-dev/devstack into tmpbranch