Define a custom SNMP group with read access only to fields we need.
For D-Link switches, modifying the group must be handled the same as
user, i.e. the group (and user) must be removed and readded.
Untested for FS S5800.
Do not try and match the global VLAN list as printed by the switch.
Instead, only try to realize the truth: there may be some VLANs added
and some removed.
We keep the compact_numlist filter and use it instead of the built-in
vlan_parser when listing VLANs for tagged ports. This is because some
switches compact 1,2,4,5,6 as 1-2,4-6 and others as 1,2,4-6 (see next
commit).
All of this should reduce the number of cases where Ansible reports a
change in configuration where there was in fact no change.
