schema: add jsonschema definition to cc_keys_to_console

Add initial jsonschema definition for validation of cloud-config
user data to cloudinit/config/cc_keys_to_console.py.

Add a schema dictionary to the cc_*.py module which describes allowed
cloud-config properties which are honored by the module.

jsonschema support for a cloud-config module should entail:
 - module-level schema dict definition in cc_*py
   - schema should contain the keys:
     id, name, title, description, distros, examples, frequency, type,
     properties
 - handler should call validate_cloudconfig_schema(cfg, schema) if valid
   top-level config module keys are provided

Good examples are:
- cloudinit/config/cc_runcmd.py
- cloudinit/config/cc_ubuntu_drivers.py
- cloudinit/config/cc_zypper_add_repo.py
- cloudinit/config/cc_ntp.py