update couchdb::add_user to use couchdb::document (#4624)

Change-Id: Ie7810d7e851f5023bc8970cccc888eccb8661856
restore missing couchdb_host variable for leap_mx

Change-Id: I5b21471ab5de55b627a200b256168122f49c1e28
replace couch-doc-update with one that uses MultiJson instead, this could prevent the outdated json errors (#4622)

Change-Id: Ibd929045a28eef1e9262fad8953df8fd150c1966