Copy tempest.log to logs dir after tempest run

Devstack change I91f2ee781c4a7a18c561ea3e3b26832b97431464 configures
tempest to print all log messages into a tempest.log file located in
the tempest repo that gets cloned as part of devstack. This commit
copies the log file into log directory so that it will get uploaded
to the log server with the other logs.

Change-Id: I45e01c87547c65725d25d8d338c983ce9297bf20