ansible_freeipa_module: New function load_cert_from_str

For certmapdata processing in ipauser it is needed to be able to load a cert
from a string given in the task to be able to get the issuer and subject of
the certificate. The format of the certifiacte here is lacking the markers
for the begin and end of the certificate. Therefore load_pem_x509_certificate
can not be used directly. Also in IPA < 4.5 it is needed to load the
certificate with load_certificate instead of load_pem_x509_certificate. The
function is implementing this properly.