Fail the step when a clone fails with no recourse

Fixes #2671.  There are a lot of issues with git's error handling..
Fix failures where 'git clean' fails but could be clobbered

Fixes #2677.
add whitespace to avoid failure with Sphinx-1.2.1
Merge djmitche/buildbot:8/git-clone-failure (PR #1057)
Merge djmitche/buildbot:8/git-clean-fails (PR #1058)
Add systemd service unit for master.

This assumes an existing user called buildbot and the master config located in /srv/buildbot/master.

Signed-off-by: David Gstir <david@nod.at>
Add systemd unit for slave.

This assumes an existing user buildslave and the slave config located in /srv/buildslave/linux-slave.

Signed-off-by: David Gstir <david@nod.at>
Merge sigma-star/buildbot:systemd-units (PR #1059)
fix typo in comment