Allow non-root ip in subnet CIDR

If a subnet is created using an IP that isn't a root IP
(doesn't end with .0) the code that handles IP checking
for fixed IPs fails.  This fix changes the check for a
valid IP to use the net.network IP address to
avoid problems in subnets created with the non-root CIDR.

(fixes bug 1188845)

Change-Id: I89df64261d0f2741668576e45dc8f026857f20b0
Merge "Allow non-root ip in subnet CIDR"