htools: add option to control the use of RAPI

RAPI needs the curl module, which is a binding to the libcurl C
library. This is more complex (can be compiled either against gnutls
or openssl, etc.) and it's not of much help when the tools are used on
the cluster itself, we disable it by default.

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