rgw_log, rgw_file: account for new required envvars

rgw_log, rgw_file: account for new required envvars

rgw_log_op() expects more server and request variables from the
request environment.  Fix rgw_lib to set more that it can
usefully, but also make rgw_log_op() check for the existence
of all keys it will use.

Fixes: http://tracker.ceph.com/issues/21942
Signed-off-by: Matt Benjamin <mbenjamin@redhat.com>