Add ipa_pki_retrieve_key_exec() interface
The ipa_pki_retrieve_key_exec() interface is needed to allow other
domains execute ipa-pki-retrieve-key.
Related: https://pagure.io/freeipa/issue/8488
Signed-off-by: Zdenek Pytela <zpytela@redhat.com>
Reviewed-By: Christian Heimes <cheimes@redhat.com>