FloatingIP PTR record functionality

* Central methods update_floatingip, get_floatingip, list_floatingip
* API according to
* https://wiki.openstack.org/wiki/Designate/Blueprints/Reverse
* Put network functionanlity into a pluggable api class (In case we want
* more)

blueprint floating-ip-ptrs

Change-Id: Ic9194e5bcfc8d25126b9e84652144f58cddcccfe
Merge "FloatingIP PTR record functionality"