Add a new command to export Ceph information from one or
more Heat stacks to be used as input of another stack.
Creates a valid YAML file with the CephExternalMultiConfig
parameter populated.
Also have export's export_password use yaml.safe_load in
place of the deprecated yaml.load.
Closes-Bug: #1895034
Change-Id: Ibdf9115e92c6b476b99d6df785b0c7e9f23991de