Please see race details in the bug report.
The fix is to check for dvr routers that needs to be removed
after actual port deletion.
As a bonus we can get rid of dvr logic in ml2 plugin delete_port().
It's hard to come up with a test here as race can only be
reproduced with multiple API workers.
Closes-Bug: #1538163
Change-Id: I7ab1b0c18e5daa6f35a37370b9be79e689e2ba50