The certificate setter function like nm_setting_802_1x_set_ca_cert()
actually load the file from disk, and validate whether it is a valid
certificate. That is very wrong to do.

For one, the certificates are external files, which are not embedded
into the NMConnection. That means, strongly validating the files while
loading the ifcfg files, is wrong because:
 - if validation fails, loading the file fails in its entirety with
   a warning in the log. That is not helpful to the user, who now
   can no longer use nmcli to fix the path of the certificate (because
   the profile failed to load in the first place).
 - even if the certificate is valid at load-time, there is no guarantee
   that it is valid later on, when we actually try to use the file. What
   good does such a validation do? nm_setting_802_1x_set_ca_cert() might
   make sense during nmcli_connection_modify(). At the moment when we
   create or update the profile, we do want to validate the ...