Specify where to obtain cert for standalone hammer

[aneta-petrova]

non-blocking:
Installing standalone hammer also has this as a prerequisite

foreman-documentation/guides/common/modules/proc_installing-standalone-hammer.adoc

Line 11 in cddb86f

* Ensure that the CA certificate of {ProjectServer} is deployed to the truststore on the host.

Now that we figured out a way how to actually get it, should we mention it there as well?

Originally posted by @adamruzicka in #4425 (comment)

[ekohl]

On my phone now, but I have some recollection Hammer has a sort of TOFU to get the certificate. https://github.com/theforeman/hammer-cli/blob/master/lib/hammer_cli/ca_cert_fetcher.rb is at least a source that implies it can just fetch it itself. Could be worth checking the source if it can be optimized using this new endpoint.