test: fix docker image checking
test: fix docker image checking

Summary:
* Take into account the image tag when checking to make
  sure it's available locally.

Task: None

Test Plan:
stepped through the code in the debugger +
ran test and made sure ubuntu:14.04 got downloaded

Reviewers: cwallin

Reviewed By: cwallin

Subscribers: mf186042, anu, akshat, rschlussel

Differential Revision: https://phabricator.td.teradata.com/D757

Svjard/presto-admin,Svjard/presto-admin,prestodb/presto-admin,prestodb/presto-admin