Get the certificate from freeIPA

@StephaneGerardVUB wrote in his documentation (http://mon.iihe.ac.be/trac/t2b/wiki/QuattorFreeIPA) : 
The goal of these commands is to extract the host certificate and key and the IPA CA certificate from nssdb, and to copy them in the /etc/ipa/quattor/certs directory.