dcmgr: add evacuate API to /host_nodes endpoint.
mussel: add "host_nodes evacuate" operation.
rewrite HA scheduling method.
handle host_node's enable flag during instance evacuation.
instance ha: support external monitoring script for failure node detection.
tidy Sequel classes up.
monitor targets including offline host node but still enabled.
mussel: add or_fail_with option to retry functions.

or_fail_with allows to quit retry loop when the given condition finds
fatal error case which is worth to cancel rather than continue the loop.
mussel: show retry loop message to stderr.
mussel: add experimental HA test. they are expected to run on VMWare.
add DB migration.
Merge branch 'feature-nfs-volume' into feature-instance-ha
dcmgr: change migration file number.
allow over commit for HA instances.
mussel: remove a unused option from "host_node evacuate".
add custom messaging client.
Merge pull request #342 from axsh/feature-instance-ha

Instance HA