Make Yardstick log level can be set by CLI

Make Yardstick log level can be set by CLI

1. Currently, the log level of yardstick is DEBUG and cannot be modified.
2. Make it can be changed from CLI.
3. When use dovetail run --debug, can see all Yardstick logs (including DEBUG)
   in dovetail.log.
4. When run without debug option, cannot see Yardstick debug logs in
   dovetail.logs.
5. yardstick.log always doesn't include debug logs.

JIRA: DOVETAIL-506

Change-Id: Iaafb61232a22e16fff2fa4605bf7e262b022085e
Signed-off-by: xudan <xudan16@huawei.com>