[lib] Create veths using systemd opnfv-fuel units

[lib] Create veths using systemd opnfv-fuel units

Create 2 systemd services on the jumphost that will handle veth
pairs creation, respectively adding them to virsh/real bridges.
This allows us to set docker containers restart policy to 'always',
enabling persistent Salt Master/MaaS containers across jumphost
reboots.

NOTE: libvirt creates virtual networks async, hence the need for
retrying hooking veths to them.

JIRA: FUEL-406

Change-Id: I1ca033cb5eb854b577b57bb2387a58bd9605a5bb
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>