Mention a way to not set tenant in midonetrc

Explain how to run CLI without any tenant loaded with midonetrc
in the man page.

Change-Id: I4ac970f1365fb5d29f2c5e7ff2360698c5db6803
Fixes: MN-2976
Signed-off-by: Ryu Ishimoto <ryu@midokura.com>