fix(runtime/networkdriver): don't fail on bridge

This patch is untested but based on a user's feedback that docker 0.9.0 was
failing to restart. The backtrace from docker looked like this:

``` 
Started Docker Application Container Engine.
[/var/lib/docker|3500bf9c] +job serveapi(fd://)
[/var/lib/docker|3500bf9c] +job initserver()
[/var/lib/docker|3500bf9c.initserver()] Creating server 2014/03/31 22:06:15
Listening for HTTP on fd ()
[/var/lib/docker|3500bf9c] +job init_networkdriver()
[/var/lib/docker|3500bf9c.init_networkdriver()] creating new bridge for
docker0 Error creating bridge: file exists
[/var/lib/docker|3500bf9c] -job init_networkdriver() = ERR (1) Error creating
bridge: file exists
[/var/lib/docker|3500bf9c] -job initserver() = ERR (1) 2014/03/31 22:06:15
Error creating bridge: file exists
```

I can write a test case for this but before I got too far along I wanted to 
make sure that ignoring EEXIST was a reasonable approach.