Allow config.floating_ip_pool to take an array as input

[julienvey]

both syntax would be accepted

os.floating_ip_pool = 'PublicNetwork1'

os.floating_ip_pool = ['PublicNetwork1', 'PublicNetwork2'] 

with the second syntax, we would try to allocate or use an IP from one or the other

[ggiamarchi]

Expected behavior :
When resolving floating IP, if an IP is already allocated in any of the pools from the list we take it. If no IP is eligible from the ones already allocated, we try to allocate one from the first pool from the list. If it succeed we take it, if it fails for any reason we try the second one. And we repeat that until we get an IP or until we reach the end of the list. If we finally cannot allocate a floating IP we will fail.

[simonmazas]

I take this one