XenAPI: move Neutron plugin install to domU

Use the same mechanism as used for nova plugin installation, thus
avoiding the need for variables in dom0. This change will also help to
move XenServer CI to neutron in the future, as the dom0 part of the
script is not executed there.

With this change, the neutron related network will always be created and
attached even if the user wants to use nova-network.

Change-Id: I8669c94e9f0aacce2990469dbabde1ff702a2769
Merge "XenAPI: move Neutron plugin install to domU"