Add variable to specify additional parameters for the Neutron service.

This patch adds a new variable, Q_SRV_EXTRA_DEFAULT_OPTS, which can
override any parameter of the [DEFAULT] section of the neutron.conf
file. It can be used for example to define the number of DHCP agents
that should host a network:
  Q_SRV_EXTRA_DEFAULT_OPTS=(dhcp_agents_per_network=2)

Change-Id: Ic34164457e64dec6ca8ec76be1772f998d100991
xenapi: preinstall python netaddr

On our CI system, the devstack instance was reporting errors, that are
related to devstack's address_in_net function - that requires the python
netaddr library to be installed. This patch preinstalls this python
package.

Fixes bug 1097667

Change-Id: I8af199427f06cfdf0a68d96d87fe3e541199dca7
Merge "xenapi: preinstall python netaddr"
Merge "Add variable to specify additional parameters for the Neutron service."