CI Updates

Description

This MR updates CI to pull various containers from containers.ligo.org instead of Docker Hub.

Also temporarily disables the building of Python3 EL7 RPMs whilst the transition from python34 to python36 is performed.

API Changes and Justification

Backwards Compatible Changes

• This change introduces no API changes
• This change adds new API calls

Backwards Incompatible Changes

• This change modifies an existing API
• This change removes an existing API

Review Status