Add DockerCli IsTerminal() check for STDOUT

`docker events > /tmp/out` should not print control
characters to non-terminal STDOUT.

This addresses commit 26b4a4920adca614f6be17a96f254f331271faf0
without creating regression described in issue #6509.

Signed-off-by: Vojtech Vitek (V-Teq) <vvitek@redhat.com>