Handle missing units values in mongodb data.

Provide a units value if it's missing in the existing mongodb data. This
is for the backward compatibility issue when the data had been already
stored before the counter_unit field was added in our meter definition.

Fixed bug #1098603.

Change-Id: I1ff3e325dc9b226e4441de6f5f9118fced67cf33
Merge "Handle missing units values in mongodb data."