[FEATURE] Added addTopic command

Added a new command to add a TopicRemote for all Gerrit repositories.

Just execute:

```
./flow gerrit:addtopic my-topic
```

This will add an remote 'gerrit/my-topic' to every Gerrit
repository in your Project you then can push your changes
to

Change-Id: Id143d588dc53cee366be78daaaef045f73d03dfe