Add some recommendations about database

Some hopefully helpful remarks about database backends

Change-Id: Ifd6ee343665717105cdda648eca072340f9b7a00
Set hbase table_prefix default to None

...table_prefix default value was empty string which fails the connection to hbase thrift server. it should be set to None instead...

Change-Id: I1339c7dad09d311f0a4e3b16d67057e3471e8c5b
Fixes: bug #1173205
Merge "Add some recommendations about database"