Add support for legacy client config

Summary

After renaming the library to liblinkahead, we should add support for the legacy client config filenames, and test them.

• Add support for existing caosdb-client.json files, and env vars
• Provide deprecation warnings
• Add tests for legacy client config

Expected Behavior

Old client config files should continue to work but provide a warning.

Actual Behavior

Steps to Reproduce the Problem

Please describe, step by step, how others can reproduce the problem. Please try these steps for yourself on a clean system.

Specifications

• Version: 0.3-dev after renaming (#94 (closed), !55 (merged) )
• Platform: any

Possible fixes

Do you have ideas how the issue can be resolved?