Enable cross testing with python-jenkins

This allows us to spot things that may be broken
by changes made to dependencies.

Adds zull job that tests tips of both projects.

Adds 'tips' tox environment that can be used by
developers to perform the same kind of testing
locally.

Change-Id: I35dbdf6acab1062109dc4b1f1474decd75932504
Signed-off-by: Sorin Sbarnea <ssbarnea@redhat.com>

.zuul.yaml

@@ -0,0 +1,14 @@
+- job:
+    name: jjb-tox-cross-python-jenkins
+    description: Tests compatibility with master branch of python-jenkins
+    parent: tox-py27
+    required-projects:
+      - openstack/python-jenkins
+      - openstack-infra/jenkins-job-builder
+    voting: false
+    failure-message: WARNING
+
+- project:
+    check:
+      jobs:
+        - jjb-tox-cross-python-jenkins

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-minversion = 1.6
+minversion = 2.2
 envlist = docs, pep8, py34, py35, py36, py27, cover
 skip_missing_interpreters = true
 
@@ -21,7 +21,19 @@ commands =
     - find . -type f -name "*.pyc" -delete
     - find . -type d -name "__pycache__" -delete
     python setup.py testr --slowest --testr-args='{posargs}'
-whitelist_externals = find
+whitelist_externals =
+    bash
+    find
+
+[testenv:tips]
+# tests what happens with unreleased version of dependencies, like python-jenkins
+install_command = pip install -U {opts} {packages}
+ignore_outcome = true
+commands =
+    bash -c "if [ -d {toxinidir}/../python-jenkins ]; then \
+    pip install -q -U -e 'git+file://{toxinidir}/../python-jenkins#egg=python-jenkins' ; else \
+    pip install -q -U -e 'git+https://git.openstack.org/openstack/python-jenkins@master#egg=python-jenkins' ; fi "
+    python setup.py testr --slowest --testr-args='{posargs}'
 
 [testenv:cover]
 commands =