Need to describe keystone endpoints and credentials for Mistral API service

[Timur Nurlygayanov]

Steps To Reproduce:
1. Clone Mistral from the repository: git clone https://github.com/stackforge/mistral
2. Install dependencies:
    apt-get update
    apt-get install gcc python-dev
    cd mistral
    tox
3. Copy config file and fix it: cp etc/mistral.conf.sample etc/mistral.conf
4. Run Mistral API service:
tox -evenv -- python mistral/cmd/api.py --config-file etc/mistral.conf
5. Send GET request to the 8989 port

Observed Result:
Error: 401 Unauthorized

[Timur Nurlygayanov (tnurlygayanov)]

Fixed in https://review.openstack.org/#/c/62037 - need to merge and retest.