Prevent creating a single IP address sized network (network details)

The backport for bug #1102504 was incomplete and did not cover
providing the same error message when creating a subnet from the
network details page.

This additional patch is not needed on the master branch, as subnet
creation was refactored during the grizzly cycle.

Fixes bug #1149890

Change-Id: I451e37c347a6540825ad9bf66d4a32c39601dd19
Merge "Prevent creating a single IP address sized network (network details)" into stable/folsom