Move external docs URLs

Signed-off-by: Zack Cerza <zack.cerza@inktank.com>
Fix incorrect exception name in docstring

Signed-off-by: Zack Cerza <zack.cerza@inktank.com>
Add a retry to the rbd volume verification

This was running into an auth problem intermittently that Josh Durgin
said is likely not a real problem. Still, try the original call first
and fall back to a different one if it fails.

Signed-off-by: Zack Cerza <zack.cerza@inktank.com>