Changes to NVP plugin configuration file

Tweak devstack support to the nicira plugin in order to
reflect changes made in:

https://review.openstack.org/#/c/26427

This complements fixes for bug #1121605

Change-Id: I221466fb40412ceb457d3e888767291a76176aa6
Check for .stackenv

* Skip sourcing .stackenv if it doesn't exist
* Remove .stackenv at end

Change-Id: Icc2e8e4ea6fada5f9d0b906fedacdbe2c1b3b320
Merge "Changes to NVP plugin configuration file"
Merge "Check for .stackenv"