Brick: Fix race in removing iSCSI device

Brick: Fix race in removing iSCSI device

Some folks are seeing race conditions on overloaded
or slow machines, where the time between issuing the
remove volume command, and the volume being from
/dev/disk/by-path by the kernel isn't long enough.

This patch adds a simple verify loop for iSCSI only.
It will try 3 times and wait 2 seconds between tries.

Also, This patch will only fix attaches that happen
in cinder for copy volume <--> image operations.
Nova needs a separate patch to fix any potential
issues there.

Change-Id: I5db9744da6b865b7d903503db8a895b409f32f5c
Closes-Bug: 1437441