Converted additional leafrefs into leafref tuples

[karthik-sethuraman]

• fixed the remaining object-references that were previously described using single leafref to now use a grouping that contains tuple of leafref statements - one for each ancestor node of the referenced node
• updated the .tree files

[y-higuchi]

👍 ONOS yang-tools was able to parse this.

[bartoszm]

looks good to me