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.

Some multiline doc fixes and the usage of os.path.join

Adds multiline docstrings fixes for the manifest module functions.

Also replace the usage of '+' with os.path.join function.