Metadata: use instance objects instead of dictionary

Commit 8ad28f924c7ff4c932bd280032cf7c8ffe5aa20a added object
support to the metadata service. We can now use an instance
object format instead of dictionary.

Change-Id: Ibe6a1e68019ad59cb57c2e406ca38fdabb8eedec
Metadata service: remove check for the instance object type

The method now receives an instance object. There is no longer
any need to convert it to an instance object.

Change-Id: I87fc41cc9dd2c2576a75f343817558849f6f1413
Metadata service: make use of get_instance_availability_zone

Commit 488fcb4ad345bfb4fd614a140b317b606e008872 added a helper
for getting the AZ for a instance. This patch makes use of that
helper method.

Change-Id: Ic01b4d8d0f0100f735f675a385134cdfff5faa85
Make ec2/cloud.py use get_instance_availability_zone helper

Commit 488fcb4ad345bfb4fd614a140b317b606e008872 added a helper
for getting the AZ for a instance. This patch makes use of that
helper method.
Remove unused conductor parameter from get_host_availability_zone()