Artifact cache configuration should have certs and keys expanded

Summary

The configuration for client-key, client-cert and server-cert do not expanduser(), which means a key like "~/.config/buildstream-client.cert" will never be found

Steps to reproduce

• Add an artifact cache with a client-cert like "~/.config/buildstream-client.cert".
• create the key at the correct location
• Watch the remote failing to initialize because the key doesn't exist.

What is the current bug behavior?

~ is not expanded for paths

What is the expected correct behavior?

~ should be expanded

Relevant logs and/or screenshots

Possible fixes

Other relevant information

• BuildStream version affected: /milestone %BuildStream_v1.x

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