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.

Closes-Bug: #1301432
Change-Id: I65fc94d3eaaade3d1402d1c82d2c1edfa7133d5a
Merge "Enforce required config params for ODL driver"