Enforce required config params for ODL driver

Raise a config error during initialization if
there is no URL, username, or password specified
in the config for the OpenDayLight ML2 driver.

Also ensure core plugin deallocation after every
test [1] following the previous backport [2].

[1] https://review.openstack.org/#/c/92793/
[2] https://review.openstack.org/#/c/102329/

Closes-Bug: #1301432
Change-Id: I65fc94d3eaaade3d1402d1c82d2c1edfa7133d5a
(cherry picked from commit 2da351af7d2a10a3055020d572d357c56ba2689b)
Merge "Enforce required config params for ODL driver" into stable/icehouse