display number of index partitions on index UI page

It would be very useful to display the number of partitions per node for a partitioned index. A lot of times, users get confused about how many partitions actually exist for an index and where are those hosted.

This information is already been returned from the /getIndexStatus REST API as detailed here:
https://issues.couchbase.com/browse/MB-29693?focusedCommentId=330833&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-330833

PartitionMap contains the details of which partitions are on which node. We only need to display the count per node.

Sample output for a partitioned index with 8 partitions residing on 2 index nodes:

"partitionMap":{"10.100.172.48:9000":[5,3,6,2],"127.0.0.1:9001":[1,7,4,8]},