Fix google-api-python-client installation in dockerfiles

[jtattermusch]

Fixes C# breakage on master:
https://fusion.corp.google.com/projectanalysis/current/KOKORO/prod%3Agrpc%2Fcore%2Fmaster%2Flinux%2Fgrpc_basictests_multilang

I've rebuilt C# dockerfiles recently, and they are now failing because google-api-python-client doesn't work without also installing oauth2client. We will likely see similar problem for other languages soon if we don't fix it.

Example error:
https://source.cloud.google.com/results/invocations/eef719c8-51a7-4110-ab4a-f5b0262e7398/targets/github%2Fgrpc%2Faggregate_tests/tests

Traceback (most recent call last):
  File "tools/run_tests/run_tests.py", line 1873, in <module>
    build_only=args.build_only)
  File "tools/run_tests/run_tests.py", line 1827, in _build_and_run
    upload_results_to_bq(resultset, args.bq_result_table,
NameError: global name 'upload_results_to_bq' is not defined
  adding: reports/ (stored 0%)
  adding: reports/index.html (deflated 22%)
+ DOCKER_EXIT_CODE=1
++ mktemp
+ TEMP_REPORTS_ZIP=/tmp/tmp.H4Cvytyhnv
+ docker cp run_tests_715fc6da-e36f-4fad-9008-0bc2e13aec3c:/var/local/git/grpc/reports.zip /tmp/tmp.H4Cvytyhnv
+ unzip -o /tmp/tmp.H4Cvytyhnv -d /tmpfs/src/github/grpc
Archive:  /tmp/tmp.H4Cvytyhnv
  inflating: /tmpfs/src/github/grpc/reports/index.html
+ rm -f /tmp/tmp.H4Cvytyhnv
+ docker rm -f run_tests_715fc6da-e36f-4fad-9008-0bc2e13aec3c
run_tests_715fc6da-e36f-4fad-9008-0bc2e13aec3c
+ exit 1
Traceback (most recent call last):
  File "tools/run_tests/run_tests.py", line 1577, in <module>
    env=env)
  File "/usr/lib/python2.7/subprocess.py", line 186, in check_call
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command 'tools/run_tests/dockerize/build_docker_and_run_tests.sh' returned non-zero exit status 1

[grpc-testing]

****************************************************************

libgrpc.so

     VM SIZE        FILE SIZE
 ++++++++++++++  ++++++++++++++

  [ = ]       0        0  [ = ]


****************************************************************

libgrpc++.so

     VM SIZE        FILE SIZE
 ++++++++++++++  ++++++++++++++

  [ = ]       0        0  [ = ]

[grpc-testing]

[trickle] No significant performance differences

[jtattermusch]

CC @stanley-cheung

[grpc-testing]

Objective-C binary sizes
*****************STATIC******************
  New size                      Old size
 1,965,335      Total (=)      1,965,335

 No significant differences in binary sizes

***************FRAMEWORKS****************
  New size                      Old size
10,888,225      Total (<)     10,888,235

 No significant differences in binary sizes

[stanley-cheung]

@jtattermusch Ack

[grpc-testing]

[microbenchmarks] No significant performance differences

[jtattermusch]

FTR google/import-mailbox-to-gmail#41