Add a keystore password for Zuul

An upcoming change to Zuul will require this password to be set
in the config file.  Prepare for it.

http: //lists.zuul-ci.org/pipermail/zuul-announce/2021-April/000090.html
Change-Id: I0ebd33c79766d5c8f0304ed0ad940b88b0fcf94a