Add return value to some network API methods

Add return value to api methods add_fixed_ip_to_instance() and
remove_fixed_ip_from_instance(). This allows the @refresh_cache
decorator to use returned nw_info model for updating instance
info cache instead of needing to make another rpc call to get
that same information.

Tests are added to make sure return values are used by the
refresh_cache decorator for methods where the rpc call returns
with the necessary nw_info model.

Change-Id: I748e4db9de62d82e4dbcb0b9c444b16dabf0ec5d
Partial-Bug: #1284345
Fix anti-affinity server-group boot failure

When the second instance is booted to an anti-affinity group
the scheduler fails with an exception due to a missing type conversion.

Closes-Bug: #1301982
Change-Id: I87b2a8d8c7d04b781c789374d6e9c7f4c8567e5d
Merge "Add return value to some network API methods"
Merge "Fix anti-affinity server-group boot failure"
Merge commit 'refs/changes/38/84138/2' of git://gold.eng.hq.xensource.com/git/github/openstack/nova into tmpbranch