Split floating ip functionality into new file.

This is to prepare for moving this code into network.api. There was
one minor change made to associate_floating_ip. It no longer checks
access to the fixed ip's network before associating the floating ip
to the fixed ip. The calling side was already verifying access to the
instance so this is unnecessary. A note was added to the method call
to ensure future usage of associate_floating keeps this in mind.

Part of blueprint optimize-nova-network

Change-Id: I4fcda82eebee46d167e028bb68c8fedf2c3e03a1
Merge "Split floating ip functionality into new file."