ec2rotatehosts: add --instance argument

allows replacement of single host without relying on ASG order
add check to warn when no hosts to rotate
instance being rotated no longer counted in ELB healthy checks, which allows
data nodes to be upgraded (ie: decommission, rotate, join, terminate)
Merge pull request #36 from WrathOfChris/cm-rotatesingle

ec2rotatehosts: add --instance argument