Provide support for kolla dev mode in mistral. When
'kolla_dev_mode' or 'mistral_dev_mode' variables are
enabled, source code of mistral project is cloned
and bind mounted.
Partially implements: blueprint mount-sources
Co-Authored-By: chenqiaomin <chen.qiaomin@99cloud.net>
Change-Id: I6e5f9b1a6b35dbb672905987e715cae4bf10af7f