Add optional env variables for customising the server process

[vizog]

This PR adds these environment variables which are read by Main:

1. S3MOCK_PORT: Changes the default (8001) port on the server
2. REPOSITORY_PATH: If set to an address, the server creates the repository on the given path rather than creating a temporary file. This helps to persist the files when the docker container restarts.
3. BUCKET_NAME: Enables automatic creation of a bucket with the given name

Enables automatic creation of a bucket with environment variable

[mgladdish]

Instead of env vars, can the options be read from play-config instead? You can still set them from env vars if you need to.

[mgladdish]

Can we make the region parameterised, too?

[vizog]

@mgladdish I agree that moving the config to play config gives more flexibility but I guess it's better to create another PR for that because it involves changing S3mock.scala or providers. The actor system which picks up the configs is not started in Main class