Remove unnecessary DB call to find EC2 AZs.

Update get_availability_zone_by_host, so it doesn't need a list of services,
and remove unneeded service_get_all_by_host calls. This removes N DB calls in
the EC2 api when doing a DescribeInstances, where N is number if instances
that are being described.

Partially fix bug 1157437

Change-Id: I0050ba736828faad94549bc6e55f4ca1d05dcbe7
Merge "Remove unnecessary DB call to find EC2 AZs."