Add and document $NOVA_VNC_ENABLED for forcing VNC on

This allows multinode setups to function correctly without enabling the
unneeded n-novnc and n-xvnc services on pure compute nodes.

Change-Id: Ie02734bb598d27560cf5d674c9e1d9b8dca3801f
Closes-Bug: 1342155
Add mysql devel as testonly alongside mysql-server

Neutron needs to be able to install MySQL-python in the functional
test job, which requires devstack to install mysql devel libraries as
testonly.

As per sdague's recommendation mysql devel libraries are now installed
for nova as well.  In the future, any service that installs
mysql-server should also install mysql devel libraries for consistency.

Related-bug: #1346444

Change-Id: Ie715841e33a2c2d31cb4ac724b302eb98ed3366a
Merge "Add mysql devel as testonly alongside mysql-server"
Merge "Add and document $NOVA_VNC_ENABLED for forcing VNC on"
Merge branch 'master' of https://github.com/openstack-dev/devstack