Improve config validation

[itsdalmo]

We should consider moving sidecred.Config to a subpackage so we can import concrete types from the provides/stores and do additional validation. This is so we can catch configuration errors before deploying the config. We should also add a validate subcommand to the CLI for this purpose.

The following interfaces should cover what we need inside sidecred.Process:

type Config interface {
  Validate() error
  Requests() []Request
}

type Request interface {
  Store()       *StoreConfig
  Credentials() []*CredentialRequest
}