guide: Don't suggest specifying PKCS12 transfer password on command line
guide: Don't suggest specifying PKCS12 transfer password on command line

`openssl pkcs12` will ask for the passsword on the command line, which
is good enough for testing. Admins who know what they are doing can
figure out themselves how to use e.g. `-password file:/path` to read it from
a file if they need the conversion to be noninteractive.

martinpitt/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,martinpitt/cockpit,mvollmer/cockpit,martinpitt/cockpit,mvollmer/cockpit,martinpitt/cockpit,mvollmer/cockpit,martinpitt/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,mvollmer/cockpit,mvollmer/cockpit,cockpit-project/cockpit