rgw: set up curl with CURL_NOSIGNAL

Fixes: #4425
Backport: bobtail
Apparently, libcurl needs that in order to be thread safe. Side
effect is that if libcurl is not compiled with c-ares support,
domain name lookups are not going to time out.
Issue affected keystone.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Reviewed-by: Greg Farnum <greg@inktank.com>