ansible_freeipa_module: New function DN_x500_text

This function is needed to properly convert issuer and subject from a
certificate or the issuer and subject parameters in ipauser for certmapdata
to the data representation where the items in DN are reversed.

The function additionally provides a fallback solution for IPA < 4.5.
Certmapdata is not supported for IPA < 4.5, but the conversion is done
before the API version can be checked.