Specify Available PS Services

We have to specify or limit the services and versions that are actually available to the application. This should limit the errors and the amount of configuration files that we have to create. We should be able to do this by creating another config file with a list of dependencies or service, or create another env variable. We will probably go with the config file. This config file will be created in the application directory when we call in `pocket init`.

Make sure that we handle scenarios where user attempted to install services that are not yet available to the application or is not a dependency of a blueprint.

Scenario: User wants to install ruby but it is not available.

1.) User typed in the pocket service install ruby.

2.) Warn the user that the service is not yet available. Exit the process.

3.) List all available services.