Change RBD delete failure log level to warn

This is a recoverable issue in the backend, so we don't have to provide
the message on the error level.

Change-Id: I35711876b2c088ad28f32abd39248dc9a467d00d
Closes-Bug: #1256259
(cherry picked from commit a2f2a0e0d2f9516d86ef5988e083f70804c3977c)
Merge "Change RBD delete failure log level to warn" into stable/havana