dog: add elapsed time to measure the program is fast or not

To be compatible with OpenStack glance and cinder driver, we cannot
show elapsed time by default. Therefore, the new option -t (--time)
is added in dog command line to achieve it. For example:

dog cluster info -t
dog vdi check test -t

Signed-off-by: Ruoyu <liangry@ucweb.com>
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>