Migrating NodesInfo API to use plugins instead of singular plugin

In order to be consistent (and because in 1.0 we switched from
parameter driven information to specifzing the metrics as part of the URI)
this patch moves from 'plugin' to 'plugins' in the Nodes Info API.

Important: This breaks BWC for 1.0 a bit more than the switch already done, but is now documented in the migration document as well.