Typo: certicates=>certificates in nova.conf

The help message for cinder_ca_certificates_file and
neutron_ca_certificates_file spelled certificates wrong.

As this is user facing, we should fix it.

Fixes bug 1227780

Change-Id: If9372fffb655d21069c005351ebb593d8e16e056
Merge "Typo: certicates=>certificates in nova.conf"