Do not attempt to run authconfig if it doesn't exist.

While I'm at it, don't force installation of authconfig and firewalld.  These
are defined as default packages in the core group, which means that for all
interactive users they will always be installed.  This is true for almost all
kickstart users too.  The only ones who will be affected are those who either
specifically remove these packages themselves or use --nocore.  Those people
don't want the package to exist anyway.

(cherry picked from commit 8f9221c396cafd9b6f046770cf83e12147566ff4)
os.path.exists -> os.path.lexists when checking for authconfig.

/usr/sbin/authconfig is a symlink with an absolute path.  When looking under
ROOT_PATH, os.path.exists will always return False.  os.path.lexists will
not.

(cherry picked from commit aa606b8fbc5f6774983315c3d21eac4dbfa2535f)