OVN: introduce mac_prefix support to IPAM

OVN: introduce mac_prefix support to IPAM

Add the possibility to specify a given mac address prefix for
dynamically generated mac address. Mac address prefix can be
specified in nbdb NB_Global table, options:mac_prefix=<mac_prefix>
This patch fix a possible issue of L2 address duplication if
multiple OVN deployments share a single broadcast domain

Acked-by: Mark Michelson <mmichels@redhat.com>
Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>