Raise rather than generating millions of IPs.

Fixes bug #1163394

Formerly if we tried to create many millions of floating IPs with a
range like "192.168.2.224/2", address_to_hosts would happily iterate
over all of them (which would take a long time) and then nova-manage
would eventually crash with an OutOfMemoryError when we tried putting
all of them in a list.  Or, if the initial list wasn't quite big
enough to use all memory, it would blow up later (even more slowly)
when we tried to put them in the database via SQLAlchemy.

Now, raise exception.InvalidInput if the number of IPs is a million
or more.  (A million is erring on the side of caution.)

Change-Id: Ifc6b6a8faadc2e97e09f9f6c975e52229f705789
Merge "Raise rather than generating millions of IPs."