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>
we shouldn't have both MIDO_USER and MIDO_USERNAME to mean the same thing.  Changing this in this patch may break other people's environment, so I think we should do this in a separate patch.