New public API for mask_password ported from incubator

mask_password is used in processutils and log modules. processutils
is going into oslo.concurrency and log into oslo.log. Since we don't
want any oslo library to depend on oslo.log, we need this facility
in oslo.utils for use by both oslo.log and oslo.concurrency.

Change-Id: I7d3cda719908413cea074935eb6c4e94cb6071d3