NSX: replace strong references to the plugin with weakref ones

This is done in accordance to a refactoring done in change 92793.

Partial-bug: #1234857

Change-Id: Iaf2bc93182c01e62bb755b4e3f104f5a926b73fc
Remove unnecessary MagicMocks in cisco unit tests

Two patches that just return static data never have
assertions made on the MagicMocks generated. This
replaces the magicmocks with lambdas to make the code
easier to read and to get a minor performance gain.

Change-Id: I064c57a66a0f7a8e3080f39cf48bd4b67cf304e3
Partial-Bug: #1316401
Merge "NSX: replace strong references to the plugin with weakref ones"
Merge "Remove unnecessary MagicMocks in cisco unit tests"