Add check for minimum disk size

Add check for minimum disk size

This fixes issue 164, and changes:

  Failure: command execution error:
  Could not add os for instance instance2 on node node4:
  OS create script failed (exited with exit code 1), last lines in the log file:
  tar: ./bin/df: Cannot open: No such file or directory
  tar: ./bin/ip: Cannot open: No such file or directory
  tar: ./bin/grep: Cannot open: No such file or directory
  tar: ./opt: Cannot mkdir: No space left on device
  tar: ./selinux: Cannot mkdir: No space left on device
  tar: Exiting with failure status due to previous errors

Into:

  Failure: command execution error:
  Could not add os for instance instance2 on node node4:
  OS create script failed (exited with exit code 1), last lines in the log file:
  Device size is too small (128 MB)
  Required size is at least 256MB

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Bernardo Dal Seno <bdalseno@google.com>