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
Run neutron-debug with admin tenant in neutron-adv-test

Because neutron-debug create-probe needs admin role only, demo tenants
cannot create ports. neutron-debug is wrapped in order to run it only
with admin tenant.

Change-Id: Ib65e8639858c597345c6a5fdc0192b40f34a0300
Closes-Bug: #1269090
Merge "Skip Nova exercises if Nova is not enabled"
Merge "Run neutron-debug with admin tenant in neutron-adv-test"