shell: disable colours when in non-interactive session

When "-" special parameter is set to "i" it is assumed session is not
interactive [1]. In such case stdout should not be colour coded. The use
case applies to situation when LISA is used in automated testing
environment (like LAVA) and called from other script.

[1] https://www.gnu.org/software/bash/manual/html_node/Is-this-Shell-Interactive_003f.html



Signed-off-by: Milosz Wasilewski <milosz.wasilewski@linaro.org>