MB-11963: Use a more friendly print format

{}.format won't work for 2.6 python.

Change-Id: I04bd5203bb01d4a40e57e329cdf41e9293d0d965
Reviewed-on: http://review.couchbase.org/40664
Tested-by: Bin Cui <bin.cui@gmail.com>
Reviewed-by: Steve Yen <steve.yen@gmail.com>