Do not complete MongoDBContext with out replset

When there are multiple mongodb peers but replset has not yet been set
on the relation, do not complete the MongoDBContext. This causes the
template to be rendered incorrectly and leads to the following error:

ERROR ceilometer ValueError: Port must be an integer between 0 and
65535:

Closes-Bug: #1750639
Change-Id: Ifb9424ac5e175d788fa21de0ee332c0a13e61a9f