The default mac table size may not be sufficient for the provider bridges.
If the mac table overflows, it has been observed that ovs-vswitchd clocks
100% CPU. So this patch configures the mac table size of the provider bridges.
It defaults to 50000 if the value is not specified.
This patch also clears the ovn-bridge-mappings property from the OpenvSwitch's
default tables row's external_ids column if 'bridge_interface_mappings' param
is not set.
Change-Id: I8f392c7807c4c6a152dfa4b77c7f4350b44324c0
Closes-bug: #1779706
Closes-bug: #1774847