runroot: Avoid squashing path components with os.path.join

Although this is often a surprising behaviour, the Python documentation
is quite clear on this:

    If a component is an absolute path, all previous components are
    thrown away and joining continues from the absolute path component.

    https://docs.python.org/2/library/os.path.html#os.path.join

That means we need to ensure that we don't end up in this case.

plugins/runroot.py

@@ -51,6 +51,7 @@ class RunRootTask(tasks.BaseTaskHandler):
             else:
                 options.append(o)
         rel_path = path[len(mount_data['mountpoint']):]
+        rel_path = rel_path[1:] if rel_path.startswith('/') else rel_path
         res = (os.path.join(mount_data['path'], rel_path), path, mount_data['fstype'], ','.join(options))
         return res