rgw: ES sync: be more restrictive on object system attrs

rgw: ES sync: be more restrictive on object system attrs

This commit drops system attributes like url-keys, nfs-ganesha unix keys,
encryption keys and other system attributes when uploading metadata to ES.
This also brings in a tiny change where the attribute substring is no longer
compared and we compare the whole attribute name against the constants in
rgw_common

Fixes:http://tracker.ceph.com/issues/36233
Signed-off-by: Abhishek Lekshmanan <abhishek@suse.com>
(cherry picked from commit 3e385c47c786ad152848b9460e1d96c6b2adf265)

	src/rgw/rgw_sync_module_es.cc
conflict due to ::encode/::decode replacement in master, reverted to older version