Fix timezone issues in bosh-blobstore-client
Fix timezone issues in bosh-blobstore-client

- python-dateutils requires the tzdata package which is not already
present on ubuntu:xenial image
- need to explicitly declare that the docker image runs in UTC time to
prevent tzdata's default interactive pkg setup

[#156582073](https://www.pivotaltracker.com/story/show/156582073)

barthy1/bosh,barthy1/bosh,barthy1/bosh,barthy1/bosh