Fixed issue with undeleted instances

* Now savanna delete assigned floating ip by its id both for neutron and nova network
* Also now savanna behaves in this way: due to unexpected error in floating ip deletion savanna removes instance anyway
* Added unit-tests

Change-Id: I5d7d1e8db22f1dd2057b7ba64e02834d36d3497b
Closes-Bug: #1262783
Merge "Fixed issue with undeleted instances"