Allow variable tunable_params_maps to have defaults specified

[bpkroth]

Right now if you construct an Environment config with a $variable tunable_params, you are required to specify a value for that variable in the a --globals or CLI --config via tunable_params_map.

e.g.,

// some_env.json
{
   "class": "mlos_bench.environments.SomeEnv",
   "name": "MyEnv",
   "include_tunables": "path/to/some_env-tunables.mlos.json",
   "tunable_params": ["$some_env_tunable_params"],
   "config": {   
      ...
   }
}

// cli-config.json
{
   "environment": "some_env.json",
   "tunable_params_map": {
      "some_env_tunable_params": ["group1", "group2"],
   }
}

This reduces usability and isn't super well documented in the configs.

It might be nice to allow Environments to specify a "default" value like we do for const_args

[bpkroth]

@motus tracking our discussion here