managed_vms/endpoints: fix lint errors

[broady]

https://travis-ci.org/GoogleCloudPlatform/python-docs-samples/builds/126491131

[broady]

./managed_vms/endpoints/clients/google-id-token-client.py:24:1: I100 Import statements are in the wrong order. from oauth2client.file should be before from oauth2client.tools

[jerjou]

Fixed in #304