add support for user networks

[naorlivne]

as mentioned in #19 having user network options might be a good idea to allow inner-pod communication between containers be done with the container hostname DNS resolution.

there are 2 options to go about it I'm still undecided will be better:

1. ensure a default Nebula user network exists (create it if needed) at worker-manager boot and connect all of the containers managed by nebula to it.
2. add the option of having users create & attach their own user networks to Nebula apps.

It's really a question of customizability vs sane defaults, thoughts?

[naorlivne]

Also thinking it might be better to deprecate network_mode and to replace it networks as the only method of networking as by default they have the none\host\bridge networks that can do just about everything network_mode can and also allow custom networks as a bonus

[naorlivne]

Main feature added in 1.1.0