Split out lint check job from python 3.5 tests (#2941)
Split out lint check job from python 3.5 tests (#2941)

This commit creates a separate job in azure pipelines for running the
lint and style checks. Previously we were integrating this with the
linux and osx python 3.5 test jobs. However this had the negative side
effect of not running any testing if a patch failed the linter. The
purpose of staging is to verify the lowest common denominator first to
verify that the patch is good without launch 9 or 10 jobs in parallel.
If we verify the test pass in 1 or 2 environments and pass pylint then
we sanity check other supported environments still pass tests. However
if we only verify pylint then we'll end up bouncing PRs off CI more
frequently because people will have to run 2 times one for lint and the
second for lint and tests. This ends up costing more jobs in the long
run. To address this it's better to make sure we run lint and tests in
parallel on the first stage.
QISKit/qiskit-sdk-py,QISKit/qiskit-sdk-py,QISKit/qiskit-sdk-py