NSX: do not raise on missing router during migration step

Return None for missing router rather raising NotFound exception;
this is what the client code was expected to handle in the first
place.

Closes-bug: 1313918

Change-Id: I573770a69d0170b027fc990c29aaa87b1fc8eccc
Allow vlan type usage for OpenDaylight ml2

The initial OpenDaylight integration with Openstack did not support
vlan isolation so it was not included as a valid type. This change
adds the vlan type as allowed.

Also modified tests to include vlan as supported in the
check_segment().

DocImpact:
Use of VLANs with ML2 and the OpenDaylight mechanism driver
requires OpenDaylight Helium or newer to be installed.

Closes-Bug: #1315475
Change-Id: I52e3c9bfdc93b8786c58954beca105e7498e3f40
Fix typos in ovs_neutron_agent.py

Comments, doc strings, and log messages fixed

Closes-Bug: 1320547
Change-Id: I1cea722ef1839826bdfb12ff15226b97d2f217f9
Merge "NSX: do not raise on missing router during migration step"
Merge "Fix typos in ovs_neutron_agent.py"
Merge "Allow vlan type usage for OpenDaylight ml2"