SELinux: Always use the default if it exists on the server
https://fedorahosted.org/sssd/ticket/1513
This is a counterpart of the FreeIPA ticket https://fedorahosted.org/freeipa/ticket/3045
During an e-mail discussion, it was decided that
* if the default is set in the IPA config object, the SSSD would use
that default no matter what
* if the default is not set (aka empty or missing), the SSSD
would just use the system default and skip creating the login
file altogether