According to the bind_port specification (as summed up in its docstring),
we should not make any change when bind_port() is called, because binding
results might end up not getting committed.

To satisfy this specification, this patch moves actual device
reconfiguration to update_port_postcommit().

In addition, this will likely make it easier to improve performance of
bulk port creation in the future, see https://bugs.launchpad.net/neutron/+bug/1976270

Note that this change introduces a different retry behaviour when we fail
to configure a device.  Since bind_port() is retried several times by
Neutron, we indirectly benefited from these retries, but this is no longer
the case.  However, NGS already has an internal retry mechanism for SSH
connection errors, which should cover most network-related issues.

To sum up, errors that are no longer covered by a retry are the ones that
happen after we successfully connect to the device.  For instance, the
switch port may not exist on the device, or the device could be in an
unexpected state such as a port being currently part of an unexpected
VLAN.  This kind of errors are unlikely to be solved by retrying, so the
new behaviour should be fine and will even allow to return the error much
faster to the end-user.

Change-Id: If4ca9c58d7f30b40992d0f1aee7e915c6978e0ca