Cap compute floating IPs tests

Nova removed the os-floating-ips endpoint in 2.36 and deprecated the
associate and disassociate fip APIs in 2.44. To reflect this, this
patch caps the corresponding tempest tests to 2.35 and 2.43.

This patch also refactors some of the housekeeping classmethods in all
floating IPs tests to reduce code duplication.

Change-Id: Ic0eb61e6bdd427c320aef76a17da157d0d9524de
5 files changed