cubicweb #890453 [multisource] have a list of pyro-ns-id for an external source [deprecated]

When there are multiple cw instances for an application, it would be usefull for the multisource specification to be able to point to all the identical active sources. This would be nice so that there is a fallback when one instance goes down, or to facilitate a no-down-time-migration.

so for example we would replace

pyro-ns-id=elo

with

pyro-ns-id=elo elo2 elo3
prioritynormal
typeenhancement
done in<not specified>
load0.000
load left0.000
closed by<not specified>