gnt-instance reinstall: Allow overriding OS parameters

This allows OS installation scripts to make use of special parameters,
e.g. to retain some data on reinstallation.

The RAPI resource is not updated as it takes all parameters via the
query string and encoding arbitrary data in a query string is tricky.
The resource will need to be changed to use the POST body instead.

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