Bridge driver panics in `CreateNetwork` if called without
a prior `Config` call. This causes issues in dnet which
tries to create network using default driver configuration.
It should be valid to call `CreateNetwork` without a prior
`Config` call in which case we need to assume default driver
config.
Fixed this by properly initializing the driver config pointer.
Also introduced a `configured` bool to make sure that still
`Config` is called exactly once for the instance of the bridge
driver.
Signed-off-by: Jana Radhakrishnan <mrjana@docker.com>