[AIRFLOW-1912] airflow.processor should not propagate logging

The root logger will write to stdout. If
redirection is used
which is the case for processors and task runs
(not runners)
this can end up in an endless loop in case
propagation is True.

Closes apache#2871 from bolkedebruin/AIRFLOW-1912

[AIRFLOW-XXX] Bump version to 1.9.0rc6

[AIRFLOW-XXX] Bump version to 1.9.0rc5

[AIRFLOW-1785] Enable Python 3 tests

Enable tests under Python 3 to make sure that
tests run under Python
3.

Closes apache#2755 from Fokko/AIRFLOW-1785-Enable-
Python3-tests

[AIRFLOW-XXX] Bump version to 1.9.0rc4

[AIRFLOW-XXX] Bump version to 1.9.0rc3

[AIRFLOW-XXX] Bump version to 1.9.0rc2

[AIRFLOW-1789][AIRFLOW-1712] Log SSHOperator stderr to log.warning

Logging functionality for SSHOperator was added in
[AIRFLOW-1712] but it
only logged stdout.
This commit also logs stderr to log.warning

Closes apache#2761 from OpringaoDoTurno/stderr_in_ssh

[AIRFLOW-1789][AIRFLOW-1712] Log SSHOperator stderr to log.warning

Logging functionality for SSHOperator was added in
[AIRFLOW-1712] but it
only logged stdout.
This commit also logs stderr to log.warning

Closes apache#2761 from OpringaoDoTurno/stderr_in_ssh

[AIRFLOW-XXX] Bump version to 1.9.0rc1