common/config: unset TEST_FS_MOUNT_OPTS across config sections

common/config: unset TEST_FS_MOUNT_OPTS across config sections

TEST_FS_MOUNT_OPTS doesn't get reset before parsing next config
section, this will cause unexpected TEST_FS_MOUNT_OPTS in test,
because it can be assigned some fs-specific mount options in
_test_mount_opts, which might not be supported by the filesystem in
next config section. And MOUNT_OPTIONS is reset, I don't see why
TEST_FS_MOUNT_OPTS shouldn't be.

Also update README.config-sections to reflect this change and fix
typos (replace MOUNT_OPTIONS with TEST_FS_MOUNT_OPTS).

Reviewed-by: Xiao Yang <yangx.jy@cn.fujitsu.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>