Fix namespace exist() method

Fix namespace exist() method for it shall not be called with a root.
Also, don't run it under the namespace so garbage_collect_namespace
method can run without rootwrap and not withi a ns.

As a result of fixing namespace listing the patch also fixes the
regression introduced (bug/1294603) to loadbalancer agent respawning
haproxy due to inability to list namespaces properly.

Change-Id: I0dc4d01b0c1c04887ec6ad5766ec7c6c96903faa
Closes-Bug: #1297594
Closes-Bug: #1294603
Merge "Fix namespace exist() method"