BootstrapStandby should download rollback image during RollingUpgrade

Due to HDFS-17178, BootstrapStandby can now succeed during a RollingUpgrade. This can lead to an edge case where the bootstrapped NameNode does not have the fsimage_rollback_N file locally so it does not recognize that it is in a RollingUpgrade. This means the bootstrapped NameNode will overwrite the VERSION file during a checkpoint because it does not know that the NameService is in a RollingUpgrade.

BootstrapStandby needs to download the fsimage_rollback_N file in addition to the normal fsimage_N file so that the bootstrapped node correctly recognizes the RollingUpgrade state of the NameService.