Ensure sbin is in PATH.

Some distros (Debian) don't have sbin in PATH for non-root users.

Nova (and possibly other services) assumes that it can invoke "sysctl"
without sudo.

Change-Id: Iced21fc1378af309fb49688f9b63f2cd8383e304
Closes-Bug: #1300800
Install dib from pip

diskimage-builder makes releases to PyPI and is not part of the
integrated release. Since it's not, we don't need to consume its
master branch - rather, we can consume its releases.

Change-Id: If9297a28604612140c39dfe44e77107d1372f0bb
Specialize Zaqar's cleanup function

Instead of having mongodb specific cleanup logic in `cleanup_zaqar`,
specialize it to perform clean ups based on the driver that has been
enabled.

Change-Id: I5807a83443b87b2c8d184e0cd2d5563a649c6273
Merge "Ensure sbin is in PATH."
Merge "Specialize Zaqar's cleanup function"
Merge "Install dib from pip"
Merge branch 'master' of https://github.com/openstack-dev/devstack