Prepare services index method for use with cells

Move the api.openstack.compute.contrib.services' index method's logic
out of the openstack api layer and into the compute api layer.

The addition of the availability_zone information is already done by the
host api. For this reason it was removed from the openstack api layer.
The behaviour of the call remains the same, only the functionality has
been moved.

This patch makes the functionality automatically work in cells.

Work towards: bug #1150499

Change-Id: Ia9489f11608167aed5168ccb3b670f395f7de5f7
Merge "Prepare services index method for use with cells"