Add session for gRPC samples.

[tswast]

These samples are being excluded from the plain tests target, since
they require the gRPC libraries (which are not compatible with Python
3, yet). This means they are not running on Jenkins or Travis.

By adding another session, we will be able to run the tests for these
samples on Jenkins.

[tswast]

@waprin @jonparrott I noticed that neither speech nor bigtable tests were appearing anywhere in Jenkins. This PR plus a config change on Jenkins to add the grpc session should remedy this issue. I was able to run locally.

# swast @ tims-cloud in ~/src/python-docs-samples on git:tswast-grpc o [14:44:38] C:1
$ source testing/resources/test-env.sh && export GOOGLE_APPLICATION_CREDENTIALS=testing/resources/service-account.json && export GOOGLE_CLIENT_SECRETS=testing/resources/client-secrets.json && nox -s grpc
nox > Running session grpc
nox > virtualenv /usr/local/google/home/swast/src/python-docs-samples/.nox/grpc -p python2.7
nox > pip install --upgrade git+https://github.com/GoogleCloudPlatform/python-repo-tools.git
nox > pip install --upgrade -r requirements-python2.7-dev.txt
nox > pip install --upgrade -r speech/api/requirements-speech_grpc.txt
nox > pip install --upgrade -r speech/api/requirements-speech_rest.txt
nox > pip install --upgrade -r bigtable/hello/requirements.txt
nox > pip install --upgrade -r bigtable/hello_happybase/requirements.txt
nox > py.test speech/api -x --no-success-flaky-report --cov --cov-config .coveragerc --cov-append --cov-report=
=============================================== test session starts ===============================================
platform linux2 -- Python 2.7.6, pytest-2.9.1, py-1.4.31, pluggy-0.3.1
rootdir: /usr/local/google/home/swast/src/python-docs-samples, inifile:
plugins: cov-2.2.1, flaky-3.1.1
collected 4 items

speech/api/speech_gcs_test.py ..
speech/api/speech_rest_test.py .
speech/api/speech_streaming_test.py .
===Flaky Test Report===


===End Flaky Test Report===

============================================ 4 passed in 4.64 seconds =============================================
nox > py.test bigtable/hello -x --no-success-flaky-report --cov --cov-config .coveragerc --cov-append --cov-report=
=============================================== test session starts ===============================================
platform linux2 -- Python 2.7.6, pytest-2.9.1, py-1.4.31, pluggy-0.3.1
rootdir: /usr/local/google/home/swast/src/python-docs-samples, inifile:
plugins: cov-2.2.1, flaky-3.1.1
collected 1 items

bigtable/hello/main_test.py .
===Flaky Test Report===


===End Flaky Test Report===

============================================ 1 passed in 3.17 seconds =============================================
nox > py.test bigtable/hello_happybase -x --no-success-flaky-report --cov --cov-config .coveragerc --cov-append --cov-report=
=============================================== test session starts ===============================================
platform linux2 -- Python 2.7.6, pytest-2.9.1, py-1.4.31, pluggy-0.3.1
rootdir: /usr/local/google/home/swast/src/python-docs-samples, inifile:
plugins: cov-2.2.1, flaky-3.1.1
collected 1 items

bigtable/hello_happybase/main_test.py .
===Flaky Test Report===


===End Flaky Test Report===

============================================ 1 passed in 3.06 seconds =============================================
nox > Session grpc successful. :)

[jerjou]

Huh - in theory, the speech tests should only be skipped if the python version was > 3... was that not working?

[tswast]

The speech directory was in the "blacklist" so nox was never considering them to be sample directories. (Ditto for bigtable.)