Ceilometer uses the ctrlplane to poll snmpd on each
ironic node.
43155ed1462a8e27c9efdbb345bfc5832c50bd2f have restrict snmpd
to the internal_api subnet which breaks Ceilometer that use the
ctrlplane subnet.
This change puts the ctrlplane as default for snmpd.
Closes-bug: #1775001
Change-Id: Ia310e02d30ce037c2cc7fec146f27fbd0f8055f4