test/ceph-disk.sh: resolve symlinks before check

Make sure symlinks are resolved in command_fixture()
before compare result of which command and the current
path.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
test/mon/mkfs.sh: add check for default keyring

The auth_cephx_key check always fails if there is a default
keyring in /etc/ceph/ available. Check if keyring exists and
fail with error message if keyring file in path.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>