arrow

ARROW-4229: [Packaging] Set crossbow target explicitly to enable building arbitrary arrow repo

This change eliminates the need of:

```
# checkout the tag under a new branch name and push that branch to your fork's remote
#
# to launch a crossbow build this branch _must_ exist on your remote
git checkout -b zero-one-zero-rc0 apache-arrow-0.1.0
git push -u <your fork's remote> zero-one-zero-rc0
```

during the [release procedure](https://cwiki.apache.org/confluence/display/ARROW/Release+Management+Guide):

Usage:

```bash
python dev/tasks/crossbow.py submit \
    -r apache/arrow \
    -t apache-arrow-0.12.0 \
    -v 0.12.0 \
    -g conda -g wheel -g linux
```

Testing it...

Author: Krisztián Szűcs <szucs.krisztian@gmail.com>

Closes #3369 from kszucs/arbitrary-crossbow-repo and squashes the following commits:

c97354ed <Krisztián Szűcs> allow passing crossbow repo and branch explicitly