dockertest.dockercmd: Fix AsyncDockerCmd to log correctly

DockerCmdBase distinguish between executed and nonexecuted command
by checking self.executed. This variable is not incremented in
AsycnDockerCmd. This patch increments the self.executed in execute()

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
subtests.docker_cli.kill: Fix incorrect Output initialization

The Output class used length of the output instead of number of lines.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
subtests.docker_cli: Add logs_follow test

This test uses `docker logs --follow` and checks it works correctly.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>