Policy check for forced_host should be before the instance is created

Move the policy enforcment for force_host and force_node into the
_handle_az() method so that any violation of policy is detected before
the DB entry for the instance is created.

Fixes bug: 1220220

Change-Id: Iaf6df4f8df6cca71fcb860244ccf758d7908b8f1
Merge "Policy check for forced_host should be before the instance is created"