move all_vbucket_ids() to base class

this allows method to be used by several test suites

Change-Id: I7511b6b7d978896dd1f03e04e297d2f17c608abd
add test for failover swap rebalance

Test loads 100k items across N vbuckets and then adds a node into a 2 node cluster (without rebalance)
Next original node is failover-over and rebalance is triggered.
ex:
./pyupr -b dev -h 192.168.1.120:9000 192.168.1.120:9001 192.168.1.120:9002 -s reb

Change-Id: Iad8e95369d8e792a3f359f411c19ed67a4093767