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>
Brian Goff