Reduce nesting in _CheckVLANArguments
Reduce nesting in _CheckVLANArguments

This avoids a pylint too-many-nested-blocks warning. It also has the
happy side effect of removing some duplicated code.

No functional change.

Signed-off-by: Brian Foley <a00e555871a6a538e97f8c44307104c884f8e869@google.com>
Reviewed-by: Iustin Pop <acf83a5274f31a43af6b914614365999469c1a31@google.com>

andir/ganeti,ganeti/ganeti,mbakke/ganeti,onponomarev/ganeti,ganeti/ganeti,onponomarev/ganeti,yiannist/ganeti,yiannist/ganeti,yiannist/ganeti,andir/ganeti,mbakke/ganeti,ganeti/ganeti,andir/ganeti,mbakke/ganeti