NSX: Avoid floating IP status reset

The method for setting the floating IP status in the NSX plugin
has a flaw that causes in some cases the status to be reset.
This patch removes this bug and also protects against regression
adding status check in the unit test covering floating ip
creation.

This patch also moves the above mentioned unit test in a more
suitable place in the test same module.

Change-Id: Ib08c421ade20ce23b5c08145203a2df6a8f3efa5
Closes-Bug: #1357514
Merge "NSX: Avoid floating IP status reset"