make backup_incremental_strategy a datastore specific option

backup_incremental_strategy remained a common option even after
backup_strategy became a datastore specific option. This change moves
backup_incremental_strategy to a datastore specific option with the
appropriate decorations for deprecation.

Change-Id: I6113ae9aede6b2c9cb960320e05960df2de34566
Closes-Bug: #1354959
Merge "make backup_incremental_strategy a datastore specific option"