Few updates for the manifests modules.

Removes the `install_cert_on_server` function as it's logic is now
moved to automation_tools github repo.

Updated Sphinx Directives for all the functions.

Now the `clone` function uses a default `key_file` and `old_path` values
to fetch manifest from the robottelo.properties file.
os.path.join would be better here, slipped past last review:)