Introduce cleart command

This patch adds "cleart" command to the top level commands. "cleart"
cleart the current tenant ID pushing None to the stack of the tenant
ID. You can pop the previous tenant ID with "popt" as well as "pusht".

This fixes [MN-2384](https://midobugs.atlassian.net/browse/MN-2384).

Signed-off-by: Taku Fukushima <tfukushima@midokura.com>