This moves the initialization of the pre-defined networks to where it's
used instead of in package init.
This reason for this change is having this be populated in `init()`
causes it to always consume cpu, and memory (4.3MB of memory), to
populate even if the package is unused (like for instnace, in a re-exec).

Here is a memory profile of docker/docker just after starting the daemon of the
top 10 largest memory consumers:

Before:
```
      flat  flat%   sum%        cum   cum%
         0     0%     0%    11.89MB 95.96%  runtime.goexit
         0     0%     0%     6.79MB 54.82%  runtime.main
         0     0%     0%     5.79MB 46.74%  main.init
         0     0%     0%     4.79MB 38.67%  github.com/docker/docker/api/server/router/network.init
         0     0%     0%     4.79MB 38.67%  github.com/docker/libnetwork.init
         0     0%     0%     4.29MB 34.63%  github.com/docker/libnetwork/ipam.init
         0     0%     0%     4.29MB 34.63%  github.com/docker/libnetwork/ipams/builtin.init
         0     0%     0%     4.29MB 34.63%  github.com/docker/libnetwork/ipamutils.init
         0     0%     0%     4.29MB 34.63%  github.com/docker/libnetwork/ipamutils.init.1
    4.29MB 34.63% 34.63%     4.29MB 34.63%  github.com/docker/libnetwork/ipamutils.initGranularPredefinedNetworks
```

After:
```
      flat  flat%   sum%        cum   cum%
         0     0%     0%  4439.37kB 89.66%  runtime.goexit
         0     0%     0%  4439.37kB 89.66%  runtime.main
         0     0%     0%  3882.11kB 78.40%  github.com/docker/docker/cli.(*Cli).Run
         0     0%     0%  3882.11kB 78.40%  main.main
 3882.11kB 78.40% 78.40%  3882.11kB 78.40%  reflect.callMethod
         0     0% 78.40%  3882.11kB 78.40%  reflect.methodValueCall
         0     0% 78.40%   557.26kB 11.25%  github.com/docker/docker/api/server.init
  557.26kB 11.25% 89.66%   557.26kB 11.25%  html.init
         0     0% 89.66%   557.26kB 11.25%  html/template.init
         0     0% 89.66%   557.26kB 11.25%  main.init
```

Now, of course the docker daemon will still need to consume this memory, but
at least now re-execs and such won't have to re-init these variables.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

- Otherwise a overlay network delete after daemon restart
  will hit a nil pointer dereference while releasing the
  vxlan id

Signed-off-by: Alessandro Boch <aboch@docker.com>

- On sandbox delete, the leave and delete of each
  endpoint is performed, regardless of whether the endpoint
  is the gw network endpoint. This endpoint is already
  automatically removed in endpoint.sbLeave() by
  sb.clearDefaultGW() when the sandbox is marked for
  deletion.
- Also restoring otiginal behavior where on disconnect
  from overlay network (only connected network), it also
  disconnects from default gw network.
- Also do not let internal network dictate container does
  not need external connectivity. Before this fix, if a container
  was connected to an overlay and an internal network, it may not
  get attached to the default gw network.
- needDefaultGw() takes now into account whether the sandbox
  is marked for deletion

Signed-off-by: Alessandro Boch <aboch@docker.com>

Signed-off-by: Santhosh Manohar <santhosh@docker.com>