remove the need for WALLET_JSON env var

[MichaelBuhler]

Currently ao-process-clients uses PATH_TO_WALLET env var, so both PATH_TO_WALLET and WALLET_JSON are needed.

This actually presents a problem that the contents of the env var may be different from the contents of the wallet file.

We can just read the contents of the file instead of using the WALLET_JSON env var, simplifying the docker-compose.yml and the provider's .env file.

• Remove WALLET_JSON.
• Read the wallet from the file at PATH_TO_WALLET.
• Orchestrator will fail to boot if PATH_TO_WALLET is not set.

IMPORTANT:

This PR requires randao/orchestrator to be build and published to Docker Hub, then another commit to update the version tag in the docker-compose.yml.