Merge branch 'master' into branch-2.1

Merge branch 'master' into branch-2.1

* master:
  Fix adjustement of candidates in cluster modify
  Add a new node list field
  Fix HTTP server library handling of credentials
  Fix a typo in backend.InstanceReboot docstring
  Fix handling of 'vcpus' in instance list
  Fix checking for valid OS in instance create
  Show disk size in instance info

Conflicts:
  lib/backend.py
    docstring update, mixed version kept
  lib/cmdlib.py
    OS validity checking, branch-2.1 version kept, since the new rpc
    included all the error checking