Revert "Remove MySQL requirements"

This reverts commit c69051367069eab4dd608753e14ffc8cee75b50b.
Add libmysqlclient-dev to ubuntu requirements

Signed-off-by: Zack Cerza <zack.cerza@inktank.com>
If lock_server is not set, check_lock() no-ops

Signed-off-by: Zack Cerza <zack@cerza.org>
Abstract the lab domain (e.g. front.sepia.ceph.com)

This should make usage in other labs easier.

Signed-off-by: Zack Cerza <zack@cerza.org>
Allow omitting 'tasks' stanza in job definitions

Signed-off-by: Zack Cerza <zack@cerza.org>