configdict: T5894: add get_config_dict() flag with_pki
VyOS has several services relaying on the PKI CLI tree to retrieve certificates.
Consuming services like ethernet, openvpn or ipsec all re-implemented the same
code to retrieve the certificates from the CLI.
This commit extends the signature of get_config_dict() with a new option with_pki
that defaults to false. If this option is set, the PKI CLI tree will be blended
into the resulting dictionary.