Specify Configs / Volumes when adding a service to an existing stack

[bodom0015]

After creating a stack with the wizard (which handles configuration), the user has the option to Add/Remove optional dependencies without recreating the entire stack. While this is helpful in simple cases, it creates a problem in more complex services.

A perfect example encompassing both of these problems can be seen in ownCloud + MySQL. Specifying ownCloud does not prompt for configuration, but the optional MySQL service requires the user to enter several passwords for proper functionality. Without entering a MySQL password/root password, the service can never successfully start. Furthermore since MySQL was never allocated a volume, it will not persist its contents between restarts (which seems suboptimal for a database).

[bodom0015]

Fixed by #47