Shows a warning message when deleting a container that contains data.

We cannot delete objects in the container since there may be millions
of objects in that. The batch deletion should be done in swift instead
of Horizon. The batch remove of objects function is only in swift CLI
command now. This commit replace the unclear message with a warning
message that tells the user the container is not empy.

Related bug: https://bugs.launchpad.net/horizon/+bug/1034890

Fixes bug: #1096084

Change-Id: I59b46a3535b8654734c3dae4eec916ab1b53c716
(cherry picked from commit eae19c1a7c2303e398b3348651482f49ea7508ce)
Merge "Shows a warning message when deleting a container that contains data." into stable/grizzly