Make resetswift customizable via environment

Instead of recommending to edit resetswift to replace "/dev/sdb1" with
"/srv/swift-disk", use an environment variable instead. This way I can
set SAIO_BLOCK_DEVICE=/srv/swift-disk in my .bashrc, and then when I'm
testing out changes to resetswift, I don't need to remember to edit
the modified script, nor do I end up submitting changes with the wrong
default in there.

The variable defaults to /dev/sdb1, so if you use the script unmodified
and don't set SAIO_BLOCK_DEVICE, nothing changes for you.

Change-Id: I741a8c91c2c54a4f32bc391cd794ef4206402753
Merge "Make resetswift customizable via environment"