Remove unused docker hub tags

[pdericson]

The following tags can be removed as we're no longer updating them:

• 1.5.14
• 1.5.14-centos
• 1.5.14-ubuntu
• 1.5.17
• 1.5.17-centos
• 1.5.17-ubuntu
• latest

It might be nice to retain the symbolic tags indefinitely but having to update the docker hub build settings by hand is error prone and time consuming.