Support docker-compose like default network behavior

[lisongmin]

When there is no network defined (neither network-mode nor networks) in service,
The behavior of default network in docker-compose and podman-compose are different.

Top-level networks	podman-compose	docker-compose
No networks	default	default
One network named net0	net0	default
Two networks named net0, net1	podman(--network=bridge)	default
Contains network named default	default	default

To enable compatibility between docker-compose and podman-compose, specify
default_net_behavior_compat: true under global x-podman key:

x-podman:
    default_net_behavior_compat: true

[p12tic]

Looks good, but please add an integration test. For an example, please see https://github.com/containers/podman-compose/pull/1044/files .

[p12tic]

Also, commit message should be 72 characters or less (probably best to remove by default_net_behavior_compat switch).

[lisongmin]

Done

[p12tic]

Thanks, looks great.