Make sure datapath_type is updated on bridges changed

Make sure datapath_type is updated on bridges changed

When changing datapath_type in the config, physical and tunnel bridges
do not have their datapath_type updated. Calling create() on already
created bridges should be safe as it passes '--may-exist' when adding
the bridge, which will do nothing if the bridge already exists, but
the second part of the transaction will still update things like
datapath_type.

It should be noted that ancillary bridges (like br-ex) are not
modified by this patch as datapath_type was never applied to them to
begin with.

Incidentally, the native and vsctl versions behaved slightly
differently when handling datapath_type: vsctl builds the multi-cmd
transaction with add-br ... -- set ..., so that the second cmd would
actually complete. The native just bailed if may_exist and the bridge
existed. This is fixed as part of this patch.

Change-Id: Ib8bc817c7bc724d80193d0ca7af480a7ea103f77
Closes-Bug: 1532273