Add --keystone-user/group to keystone-manage pki_setup

If called as root, --keystone-user and --keystone-group can be
used to set the username and group keystone is going to run under.

In that case, pki_setup is going to issue additional os.chown
calls to change ownership of the PK files accordingly.

Fixes LP Bug #1031372

Change-Id: If9250ca9d0d86eebb9ad7c95ade17132ffd5a36c
Merge "Add --keystone-user/group to keystone-manage pki_setup"