stack/erp5: Fix master node separation in NEO URI

In nexedi/slapos@0cf70a6e we already migrated the NEO URI concatenation to the
recent format used by WCFS. Unfortunately we missed separating the address
(IPv6 + port) of the master nodes from each other. Without adding the
separator ',' between them, WCFS doesn't work with NEO clusters which have
multiple master nodes, because NEO/go can't split the different master node
addresses.