associate an uuid to yardstick_key and yardstick_key.pub

associate an uuid to yardstick_key and yardstick_key.pub

JIRA: YARDSTICK-527

This work is to support yardstick parallel tasks in the future.
Currently, the RSA key we generated and used to access the VM is named
'yardstick_key'.
If more than two tasks are running paralleled, the later 'yardstick_key' will
cover the former.
We want associate an uuid to identify differnets for each tasks. So the key
files won't conflict.
The first 8 digits will be used, as there is no need to used a full-length uuid.

Change-Id: If8eaf47ae527cf9b3bd50f37ab3051fbdccf5f03
Signed-off-by: JingLu5 <lvjing5@huawei.com>