fix SNMPv3 version identifier
drop duplicated code

The handling of mac addresses for non-default vlans has been moved in
_setAssociatedMacAddresses function
no need to pass manufacturer anymore

We try to switch SNMP context for any device failing to provide any
result for dot1qTpFdbPort.
exclude VLAN 1, it is the default context
prepare for 2.3.5 release
fix potential warnings with lspv output

See resources/aix/lspv/aix-6.1-hdisk1 for an example of output with
undefined values.

see: #2384
allow easier parameters override, for testing
prevent warning for malformed group file

see: #2384
prevent potential warning with lspv output

And simplifiy parsing by using getFirstMatch() function.

see: #2384
don't forget to use actual command if needed...
more consistency in function names and goals

Keep logical volume info extraction in _getVolumeInfo(), renamed to
_getLogicalVolume() for consistency with similar functions, rename
original _getLogicalVolume() to _getLogicalVolumesFromGroup() and limit
it to returning a list of volume names.

see: #2384
test logical volume names extraction

see: #2384
cosmetics

see: #2384