Separate configuration based on test environment

Added:
- ARTIFACTS variable to store all test artifacts
  under single directory
- yaml config generation based on environment variables
- NAILGUN_CONFIG variable in run_tests.sh and settings.py

Commands used for testing:
  TEST_NAILGUN_DB=nailgun1 ARTIFACTS=/tmp/test_runs/1 \
  UI_SERVER_PORT=8011 FUELCLIENT_SERVER_PORT=8021 ./run_tests.sh

  TEST_NAILGUN_DB=nailgun2 ARTIFACTS=/tmp/test_runs/2 \
  UI_SERVER_PORT=8012 FUELCLIENT_SERVER_PORT=8022 ./run_tests.sh

Change-Id: If03dddee9a8259a80b526a53fe61a79c544c72a4
Closes-Bug: #1282565
Closes-Bug: #1334180
Merge "Separate configuration based on test environment"