Remove import extension dep from db migration

Change-Id: Ic8907a9960807b30d8b89df0ee29e42b26a7675a
Closes-bug:1285294
Renaming plugin-specific exceptions to match NSX

Try to get rid of Nvp as much as possible, the base
class already uses the plugin short name so it's
pointless to carry it over.

Partial-implements blueprint nicira-plugin-renaming

Change-Id: Ic3e53bc4e41ffe1a19ea03e807ec89bb6c864f51
Update License Headers to replace Nicira with VMware

In the process __init__.py involved have been emptied;
vim modelines and author lines have been dropped from
the license headers affected by the change as well.

Partial-implements blueprint nicira-plugin-renaming

Change-Id: I9c535b59bb2e83f275816472e0e32b41444d3bbe
Merge "Remove import extension dep from db migration"
Merge "Renaming plugin-specific exceptions to match NSX"
Merge "Update License Headers to replace Nicira with VMware"