Add possibility to override some environment variables

Add possibility to use custom 'master_environment_override'
and 'mirror_environment_override' files, to avoid sed-ing the
user-data files.

Related issue: https://mirantis.jira.com/browse/PROD-23809

Change-Id: I664a200358f98e35ceb28267dc40c4974778ba30
2 files changed
tree: 1cb85a023e10a3bc191193bb3c96a0c6f5cb1b6d
  1. config-drive/
  2. mirror-image/
  3. predefine-vm/
  4. salt-ca/