Be more flexible with addresses in ZkOccConnection.

Allow the set of zkocc hosts to be updated. Also, limit the number of
dial attempts for any given operation - this is useful when the set of
zkocc hosts gets large.
Rename to 'refresh' since 'update' is a semantically loaded term in Python.
Merge pull request #85 from plobsing/zkocc-conn-addrs

Zkocc conn addrs
Fail fast and report errors when parsing zk config files.
Merge branch 'master' of github.com:youtube/vitess
Merge branch 'master' into replication