[autotest] test failure send email via a different gmail account

Test failures notification emails sometimes causing quota issues
when a huge amount of notifications are sent out around the same time.

Move it to a different gmail account to gurantee that more
critical emails like inventory emails can be sent out on time.

Also, add support for a "creds" dir.

TEST=send email using the new gmail account:
- with gmail_api_credentials_test_failure and creds dir set.
  confirm using new credentials.
- without gmail_api_credentials_test_failure and creds dir set.
  confirm using the default credentials.
BUG=chromium:541824;chromium:567882

Change-Id: I191e14303cdead164147d188ac495da6f7c18ba6
Reviewed-on: https://chromium-review.googlesource.com/316680
Commit-Ready: Fang Deng <[email protected]>
Tested-by: Fang Deng <[email protected]>
Reviewed-by: Fang Deng <[email protected]>
4 files changed