Inconsistent nox session names

[duggelz]

These three nox.py files have not been updated to use the new session names like unit(py='2.7'), and still use the old names like unit_tests(py='2.7')

• bigquery_datatransfer/nox.py
• container/nox.py
• dataproc/nox.py

[dhermes]

@lukesneeringer This is also a de-facto bug report for the generator.

[chemelnucfin]

The manual change is #4646,
the autogenerator change is here
https://github.com/googleapis/toolkit/issues/1768

[chemelnucfin]

Since this is now tracked in Cleanup and also is reported in the toolkit repository, I will close this issue.