pull: allow pulling groups selectively

pull: allow pulling groups selectively

without requiring workarounds based on ownership and limited
visibility/access.

if a group filter is set, remove_vanished will only consider filtered
groups for removal to prevent concurrent disjunct filters from trashing
eachother's synced groups.

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>
Reviewed-by: Dominik Csapak <d.csapak@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>