mongodb, db2: do not print full URL in logs

The full URL used to connect to MongoDB or DB2 might contains sensitive
information such as username and password, so it's better to not print
it at all.
Instead, just print the hosts that are being connected to.

Fixes-Bug: #1244476

Change-Id: I092be52a760cc97a9c3e6357d19609b8886e3175
Merge "mongodb, db2: do not print full URL in logs"