Add support for .env variables in the configuration

[basmasking]

In story #448 protected procedures are introduced. To execute these procedures, a shared secret is required. This secret is currently set in the services configuration as plain text. This is not a secure way to store the keys, so we want to be able to use environment variables in the configuration file. The config would look like:

"node"
{
    "secret": "${NAME_OF_ENV_VARIABLE}"
}

It should still be possible to set a string value in the configuration.