Travis CI: use specific Python version during build

This is a preparatory work for the future requirement of running
Python2/3 jobs simultaneously.

Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>
Reviewed-By: Fraser Tweedale <ftweedal@redhat.com>

.travis.yml

@@ -2,6 +2,8 @@ language: python
 services:
     - docker
 
+python:
+    - "2.7"
 cache: pip
 env:
     global:

.travis_run_task.sh

@@ -1,8 +1,8 @@
 #!/bin/bash
 
 # NOTE: this script is intended to run in Travis CI only
-set -ev
 
+PYTHON="/usr/bin/python${TRAVIS_PYTHON_VERSION}"
 test_set=""
 developer_mode_opt="--developer-mode"
 
@@ -29,6 +29,7 @@ docker pull $TEST_RUNNER_IMAGE
 ipa-docker-test-runner -l $CI_RESULTS_LOG \
     -c $TEST_RUNNER_CONFIG \
     $developer_mode_opt \
+    --container-environment "PYTHON=$PYTHON" \
     --container-image $TEST_RUNNER_IMAGE \
     --git-repo $TRAVIS_BUILD_DIR \
     $TASK_TO_RUN $test_set