add reuse_ips option and use it to control whether openstack builder
will attempt to reuse existing unassigned floating ips rather than
allocating a new one.
fix#4551
Rather than always creating a new floating IP, search the provided pool
for an unallocated address and use this instead. Only allocate a temporary
address if there is no spare address in the pool.
Closes#4215
Matthew Hooker
Joshua Randall
Zanetti, David
Mitchell Hashimoto
Ben Broderick Phillips