Merge remote branch 'origin/devel-2.1'

Merge remote branch 'origin/devel-2.1'

* origin/devel-2.1:
  Implement QA tests for disk template changes
  Update instance modify documentation
  Implement conversion from drbd to plain
  Implement conversion from plain to drbd
  Abstract check that an instance is down
  Abstract node free disk space check
  Abstract disk template verification
  Update documentation for disk adoption
  Implement disk adoption mode in gnt-instance
  LUCreateInstance: implement disk adoption mode
  LUCreateInstance: Move parameter init earlier
  ConfigWriter: add an LV reservation manager
  Fix two issues related to check-man
  utils.RunCmd: Test case with reset_env set and setting variables

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>