responder: make sure SSS_DP_CERT is passed to files provider
Currently the files provider is only contacted once in a while to update
the full cache with fresh data from the passwd file. To allow rule based
certificate mapping the lookup by certificate request must be always
send to the file provider so that it can evaluate the rules and add the
certificate to cached entry of the matching user.
Related to https://pagure.io/SSSD/sssd/issue/3500
Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>