Merge Sakuli logs instead of overwriting them

[svettwer]

As a user, I want the Sakuli container to merge the sakuli.log file instead of overwriting them after test suite execution, so that a concurrent container execution won't override the log of each other.

Acceptance criteria:

• The sakuli.log is extended by the log messages of the last execution
• The sakuli.log is not overwritten so that concurrent containers do not override each others logs