sssd: do not write SSSD PAM if there is no sssd.conf present
The expectation of authconfig is to write sssd.conf only if there
is no domain present and the authconfig options imply an implicit
SSSD configuration. In this case, the authconfig writes a default
domain into the sssd.conf to create a vlid configuration.
However, we broke this behaviour by introducing writeSSSDPam to write
smartcard options into sssd.conf and we end up creating an invalid
sssd.conf when the file did not exist and this is not an implicit
configuration.
Now, if the file does not exist and authconfig does not create a
domain (implicit configuration) we do not write the options into
sssd.conf to comply with expected behaviour of previous version.
If the file does exist we write the PAM option in every case so the
administrator can use authconfig to enable and disable smartcard
support for sssd.