Initial metadata server API

Initial metadata server API

This implements the basic capabilities we need from the metadata server. Each
API call returns proper HTTP responses (404, 409, etc.).

The server is currently not connected to a real database. Rather, it uses a
simple mock. This allows for quick initial API changes before things stabilize.

The immediate next steps are to integrate the server with the cfn tools
(cfn-metadata being the prime candidate) to see what may be wrong/missing.

And then to connect the server to a real database.

Signed-off-by: Tomas Sedovic <tomas@sedovic.cz>