Rename Neutron core/service plugins for VMware NSX

Short aliases are introduced for the NSX plugin(s). This patch
also switches to using the new plugin names/naming scheme in
the unit tests.

Partial-implements blueprint: nicira-plugin-renaming

Change-Id: If1f1431b098bf77b9c85e0afb3d7848083d9d451
Merge "Rename Neutron core/service plugins for VMware NSX"