add --rebuild option for ssl/pki_setup

Currently, if you run "keystone-manage ssl_setup" more than once,
it does not overwrite existing files under /etc/keystone/ssl, and
does not print or log an error message. Now, we add --rebuild option
for ssl/pki_setup to clean up the exist generated files and rebuild
the configure files. Also, if you run "keystone-manage ssl_setup"
more than once, it will print prompt message to tell you take
--rebuild option to rebuild configure files.

Change-Id: Ia1e6830c4c81d5a53317a522c526b79b67de1bce
Closes-Bug: #1308778
Merge "add --rebuild option for ssl/pki_setup"