Add Cloud model for Bool 'enabled' & floating_ip

This enabled more granular control when multiple
Cloud's are in use.  Allows per provider enable
or disable, and also select if the cloud requires
a separate floating_ip.  (The setting,
use_floating_ip - still allows maas False setting

There is an associated migration, which provides
a no-change default.

A minor logging line was also changed, to add
more verbosity to what is being spawned.