Charm Proof: Ensure the default configs match the defined type

Kapil gave the example of bool. The GUI is actually doing it wrong, AFAICT, actually. Here's an except from our config.yaml:

  staging:
    description: |
      Connect the Juju GUI to the staging backend (i.e. a simulated Juju
      environment). Currently juju-core does not support the staging
      backend. For this reason, this option is ignored if the charm is
      deployed using juju-core.
    type: boolean
    default: false

The given default is actually a string, "false," and not the boolean value false. YAML specifies a typecast (http://www.yaml.org/spec/1.2/spec.html#id2803629):

  staging:
    description: |
      Connect the Juju GUI to the staging backend (i.e. a simulated Juju
      environment). Currently juju-core does not support the staging
      backend. For this reason, this option is ignored if the charm is
      deployed using juju-core.
    type: boolean
    default: !!bool false

The larger point being, 'proof' should check to ensure the default value for an entity is of the correct type. If it is not, and is not coercible, then deploying with the default config may error. E.g.,

type: int
default: cow

should cause proof to flag an error.