Fix cidr get_attr in custom networks

Fix cidr get_attr in custom networks

We were missing the square brackets around the list of arguments
for get_attr when building the networks cidr output.

This passed CI because Heat does not fail validation and Ceph (which
is consuming the cidr output) is tested with a single network (ctlplane)
which does not build the output using the same templates.

Change-Id: I40bba0784a30295cb0d4eda1fbff20ebac85db99
Closes-Bug: #1709464