Print help message when starting tikv-ctl without argument (#3531)
Print help message when starting tikv-ctl without argument (#3531)

Signed-off-by: Xiaoguang Sun <ac8c70efc34e705c6f908e5bc079f77a75790d36@gmail.com>
c4pt0r/tikv,pingcap/tikv,BusyJay/tikv,BusyJay/tikv,BusyJay/tikv,BusyJay/tikv,pingcap/tikv