Add vendor specific data to metadata service

There was not previously any way for a vendor to extend the metadata
service to include vendor specific or site specific information. A "vendor-data" location in the metadata service can serve a couple purposes:
 * provide vendors a way to make instances aware of features or locations specific to the cloud where the instance is running.
 * provide an experimentation zone for metadata. By adding a way to easily add data to an instance metadata can be experimented with more easily, which could eventually lead to to moving of an item from vendor data to metadata.

The plan is to
 * add an entry named 'vendor_data.json' to the metadata service and config-drive rendering.
 * add a example class that reads json formated content from a configured file.

Some examples of things that a vendor might put in 'vendor-data' are:
 * information on local mirrors
 * location of a local proxy.
 * create a one time license registration codes
 * static networking routes