Adding doc8 to test-requirements
Adding doc8 to test-requirements

tox -e doc was failing due to missing doc8. With this
change doc8 is added to test-requirements.txt.

Change-Id: I5a88d2068c961245b4b9ed60948532f232b859a5
Signed-off-by: csatari <d96e7a23559ca6583545c8d8ad344206bcf45e4b@nokia.com>

openstack/mistral,StackStorm/mistral,openstack/mistral,StackStorm/mistral