example-deployment: fix broken link
api: add first pass at validation for units

Introduce ValidateOptions function which defines the valid set of
UnitOptions. This is invoked in the HTTP API during a set operation.

For now, fleetctl also performs a basic check (which will be removed
once fleetctl migrates to the HTTP API).
Merge pull request #659 from jonboulle/valid

formalize valid requirements specifications
agent: add metadata validation for global units