rgw: rework framework configuration

The framework config code is now more generic and ready for multiple
config options. Different frameworks are comma separated and each
framework's config list is separated by space. Each config value is in
the key[=value] format. E.g.,

rgw framework = fastcgi, mongoose port=8080

Fastcgi socket configurables can now be set via this config line (either
socket_path, or socket_host + socket_port), e.g.,

rgw framework = fastcgi socket_path=/var/run/radosgw/rgw.sock

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>