Make test running concurrency dynamic based on nprocs (Qiskit#1973)

* Make test running concurrency dynamic based on nprocs

In Qiskit#1228 we decreased the concurrency used for the test runner in CI to
avoid memory pressure and failing jobs. In that commit we set it to 2 as
a quick fix so we could unblock CI and continue landing patches. A
followup was to do the analysis and find the sweet spot for concurrency
and stability. At the same time we opened mtreinish/stestr#202 to add
support to the test runner for running fractional nprocs. That is still
under discussion, but in the meantime this commit adds support to the
makefile for using fractional nprocs concurrency. It adjusts are default
concurrency from 2 to be nprocs / 2 if nprocs > 3. This should enable us
to increase the number of test workers we run on travis linux jobs but
still maintain reliability.

* Hard code osx to 2 workers

* Add debug statement about core count and worker count

* Use printf to round floats to int

Makefile

@@ -3,6 +3,28 @@
 # This source code is licensed under the Apache License, Version 2.0 found in
 # the LICENSE.txt file in the root directory of this source tree.
 
+OS := $(shell uname -s)
+
+ifeq ($(OS), Linux)
+  NPROCS := $(shell grep -c ^processor /proc/cpuinfo)
+else ifeq ($(OS), Darwin)
+  NPROCS := 2
+else
+  NPROCS := 0
+endif # $(OS)
+
+ifeq ($(NPROCS), 2)
+	CONCURRENCY := 2
+else ifeq ($(NPROCS), 1)
+	CONCURRENCY := 1
+else ifeq ($(NPROCS), 3)
+	CONCURRENCY := 3
+else ifeq ($(NPROCS), 0)
+	CONCURRENCY := 0
+else
+	CONCURRENCY := $(shell echo "$(NPROCS) 2" | awk '{printf "%.0f", $$1 / $$2}')
+endif
+
 .PHONY: env lint test test_record test_mock test_ci
 
 # Dependencies need to be installed on the Anaconda virtual environment.
@@ -34,7 +56,8 @@ test_recording:
 	env QISKIT_TESTS=rec python3 -m unittest discover -s test -v
 
 test_ci:
-	stestr run --concurrency 2
+	echo "Detected $(NPROCS) CPUs running with $(CONCURRENCY) workers"
+	stestr run --concurrency $(CONCURRENCY)
 
 profile:
 	python3 -m unittest discover -p "profile*.py" -v