runltp: add -z, -Z options and change device symbol in runtest files

Add two options -z and -Z for runltp, to allow user specify big block
device for some tests, such as fs_ext4(ext4 new features tests).

We abandon the old way of assigning specific device in runltp like that
"sed -i 's|DEVICE|$DEVICE|'  ${TMP}/alltests". Now we use shell
variables directly in runtest files.

runltp will export four shell variables to indicate devices.
    LTP_DEV:     small block device(the original DEVICE).
    LTP_BIG_DEV: big block device. if user doesn't specify it,
                 LTP_BIG_DEV will be empty
    LTP_DEV_FS_TYPE: default ext2
    LTP_BIG_DEV_FS_TYPE: default ext2

Signed-off-by: Xiaoguang Wang <wangxg.fnst@cn.fujitsu.com>
Signed-off-by: Cyril Hrubis <chrubis@suse.cz>