Fix mongo db nodes collection

We should not put a comma at the end of mongo db nodes list.
Because current mongo implementation supports max 3 mongo nodes,
fix the cases for # of mongo nodes < 3.

Closes-bug: #1311088

Change-Id: I5276aa71b3a637cb485983ec13ebcd83f8add5fc
Signed-off-by: Bogdan Dobrelya <bdobrelia@mirantis.com>
Merge "Fix mongo db nodes collection"