Add a Kolla 'devstack' mode

Add a new variable 'kolla_devmode', which when enabled, clones and
bindmounts service source code into the containers.

This commit adds the relevant changes for Heat, more services can be
added and built upon.

Usage:
* Set 'kolla_devmode: yes'

* Code is cloned to /opt/stack/{{ project_name }} on target
  node(s)

* Users can develop in these repos, and simply restart the container to
  pick up / test changes.

Debugging can be done from the host via 'remote_pdb'[0].

[0] https://pypi.python.org/pypi/remote-pdb

Implements: blueprint mount-sources
Change-Id: Ic0431b10d723bf84eeefc72039376fe0058dd902