Add necessary variable defaults to Makefile

[tjmoore4]

Checklist:

• [x] Have you added an explanation of what your changes do and why you'd like them to be included?
• [x] Have you updated or added documentation for the change, as applicable?
• [x] Have you tested your changes on all related environments with successful results, as applicable?

Type of Changes:

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [x] Other

What is the new behavior (if this is a feature change)?

Allow 'make all' to run without having to define the 'CCP_IMAGE_PREFIX', 'CCP_IMAGE_TAG' or 'CCP_POSTGIS_IMAGE_TAG' environment variables.

Other information: