[ovs] Start ovs services before networking

[ovs] Start ovs services before networking

Fix broken systemd service unit dependecies:
- OVS should start before networking service;
- OVS ports & bridges should not be automatically ifup-ed by
  networking service to avoid races, so drop 'auto' for both
  (OVS ports are automatically handled when part of an OVS bridge);
- explicitly ifup OVS bridges as part of networking service, but
  after all Linux interfaces have been handled;
- use 'allow-ovs br-prv' to let OVS handle br-prv and avoid another
  race condition;

While at it, fix some other related issues:
- make OVS service start after DPDK service (if present);
- bump OVS-DPDK compute VMs RAM since since switching from MTU 1500
  to jumbo frames for virtual PODs a while ago failed to do so [1];
- avoid creating conflicting reclass linux.network.interfaces entries
  for OVS ports by using their name (drop 'ovs_port_' prefix):
  * for untagged networks they will override existing common defs;
  * for tagged networks, they will create separate entries;
- DPDK scenarios: make gtw01 br-prv members OVS ports to avoid race
  conditions after node reboot by letting OVS handle them;

[1] https://developers.redhat.com/blog/2018/03/16/\
    ovs-dpdk-hugepage-memory/

Change-Id: I0266ba67f3849b6f7e331a758146b331730bae55
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>