When creating a subnet, we retrieve the allocation_ranges from the
variables dict with a default value of "". This should then be fine for
various string operations. The problems is that ansible defaults
varialbes that aren't supplied to None. This means they are present in
the variables dict so don't get the default value of "".
To fix this the code that parses allocation_ranges is wrapped in an
additional conditional.
Change-Id: Ie0310c3da050336ed9b5f5d8fc02e3a4d704491d
41778e3bf3