Instead of only file-based persistence which is reported to
potentially expose IPMI credential by persisting the temp file(s) for a
little too long.
User can now pass ``True`` to the ``ipmi_store_cred_in_env`` parameter
which instead stores IPMI password as an environment variable,
limiting exposure to the user session of Ironic.
Defaults to ``False``.
Closes-Bug: #2058749
Change-Id: Icd91e969e5c58bf42fc50958c3cd1acabd36ccdf
483bfbcdb6