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.

Signed-off-by: Taku Fukushima <tfukushima@midokura.com>
Assign password to correct var

Signed-off-by: Joe Mills <joe@midokura.com>
Merge pull request #230 from tomoe/master

Change no tenant configuration as an empty string