Change no tenant configuration as an empty string

This patch change the mapping of the empty tenant from None to the empty
string.

This resolves [MN-2389](https://midobugs.atlassian.net/browse/MN-2389).

Signed-off-by: Taku Fukushima <tfukushima@midokura.com>
This always overrides self.tenant_id set previously from the configuration file (.midonetrc) won't it?  I think we need to check here if os.environ.has_key("MIDO_TENANT")