Add ability to disable pki_setup

The enable_pki_setup parameter adds the ability to disable pki_setup,
allowing the user to manage the signing certificate himself.

Change-Id: Icb84e01adecdd3287ceb22defb9882f03ae8d96f
Closes-bug: #1278546
(cherry picked from commit e6b2315808d633deb88246c74f4c91c1da2ccc43)