RAPI: Implement OS parameters for instance reinstallation

Dictionaries are hard to encode into query strings, therefore the
“/2/instances/[instance_name]/reinstall” resource is changed to accept
its parameters via the request body. The old query string parameters are
still accepted for backwards compatibility.

To allow clients to detect whether a server supports the new body
parameters, a new feature string is added to the “/2/features” resource.
Some people might not like this, but it reuses existing functionality.

The RAPI client and its unittests are updated.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>