Fixes nbd device can't be released error

When the 'mount' operation is successful but stderr is not empty during
injecting key/file into image, there may be some warning messages output to
stderr, so we need to discard them and continue to inject files into image.

Fixes bug #1152519

Change-Id: Icc7d15196f3ee67674f9d815ddeef4378f363e2c
Merge "Fixes nbd device can't be released error"
websockify 0.4 is busted

This was just released and it is broken, stalling our gates.  When
attempting to install it, we get:

"error: can't copy 'include/util.js': doesn't exist or not a regular
file"

Change-Id: I9d93bff92cc4a9d837382acce78bff587f52fb99
Merge "websockify 0.4 is busted"