Option for installing spice-html5 and novnc from packages

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
Merge branch 'master' of github.com:openstack-dev/devstack