fixes #3902 - [Openstack] allow selection of internal networks

This patch allows the user to select the internal network when deploying
a new instance in openstack.

In Grizlly, this was done automaticilly, with a nasty side effect, that
all internal networks would get assigned (didnt see that as i had only
one internal network when i was testing), starting in Havana, this is no
longer possible, and the vm creation is rejected.

as a bonus, we now retain the floating ip selection if the form had an
error.
Fixes #7750 - hidden required fields do not prevent submit

When a field is explicitly required an HTML5 tag required was added
which caused issues during form submit.
fixes #7737 - no specific logout URL needed, will go directly back to login.
fixes #4672 - added template_name template variable