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>
Isn't this also wrong?  Even if self.tenant_id is already set here, if MIDO_TENANT is specified you want that to override it.