The novnc and spice-html5 is installed from git repository by default,
but not from an openstack* repository.
In order to add vnc/spice proxy related gating tests they should be installed
from packages.
New boolean variables added to control the installation source:
NOVNC_FROM_PACKAGE and SPICE_FROM_PACKAGE
Related changes:
https://review.openstack.org/#/c/51790/https://review.openstack.org/#/c/50822/
Change-Id: I1e55fd99edd30876924a13160afb74ff3e97c485
5a35e73b34