Python 2.7 Kokoro run fails for a directory with no Python 2 tests

[busunkim96]

Seen in PR #2554.

Full log

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- testing run/system-package
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 Using noxfile from project root. 

Error while collecting sessions.
Sessions not found: py2-2.7

 Testing failed: Nox returned a non-zero exit code. 

cleanup

[leahecole]

We're going to have similar problems for #2618 now that I've added it to the noxfile config as well

[busunkim96]

I made a small tweak to the kokoro config to run py2 rather than py2-2.7, which I think will fix it. python2.7/common.cfg

Closing for now, but please re-open if you see this happening.

[leahecole]

This is happening for the things I added in #2618 (run/hellobroken) as well as run/imageprocessing

[andrewferlitsch]

@busunkim96 PTAL