Discourage use of pki_setup

After a mailing list discussion it is clear that users need more
information to make a decision on how to setup their PKI tokens.

This recommends an external CA over using keystone pki_setup as it does
offer a bit more protection against an intruder being able to setup a
rogue token generator.

Change-Id: I798a562dd8c222e04660482b434ed3bbe6b63d62
Merge "Discourage use of pki_setup"