SSL PEM parameter name changes & puppetdb-ssl-setup handling for pem files

This changes puppetdb-ssl-setup to assist setup of PEM based files instead
of keystore based files.

The script however tries to be delicate around legacy setups, failing to run
if the user still looks like they have a keystore setup without the -f
switch. This is so users who already have keystore based setups can choose
to migrate when they wish, instead of a package upgrade forcing them to.

Users who install from scratch however, will now have puppetdb-ssl-setup
copy the certificates from Puppet into the correct places for them and
configure SSL as before, but instead with PEM files.

This patch also changes the names of parameters from the original patch to
remove the puppet-agent-* prefixes.

Documentation that relates to puppetdb-ssl-setup has been modified to suite
the new behaviour.

Signed-off-by: Ken Barber <ken@bob.sh>