Move exception definitions out of db api

The two exceptions, NoMoreNetworks and NoMoreTargets, are defined
in nova/db/api. While NoMoreTargets is unused and can be removed,
NoMoreNetworks are still used and should be moved to exception.py
where all other exceptions live.

This came up during review of https://review.openstack.org/#/c/42980/
where compute manager would need to import nova.db to make use of
NoMoreNetworks.

Change-Id: I57f0a081d2fea35394421932ae7d2a3e67540a31

nova/api/openstack/compute/contrib/cloudpipe.py

@@ -23,7 +23,6 @@ from nova.cloudpipe import pipelib
 from nova import compute
 from nova.compute import utils as compute_utils
 from nova.compute import vm_states
-from nova import db
 from nova import exception
 from nova import network
 from nova.openstack.common import fileutils
@@ -146,7 +145,7 @@ class CloudpipeController(object):
             try:
                 result = self.cloudpipe.launch_vpn_instance(context)
                 instance = result[0][0]
-            except db.NoMoreNetworks:
+            except exception.NoMoreNetworks:
                 msg = _("Unable to claim IP for VPN instances, ensure it "
                         "isn't running, and try again in a few minutes")
                 raise exception.HTTPBadRequest(explanation=msg)