Change few attributes to be unset
Change few attributes to be unset

This patch enables users to unset the following attributes:

* Router's load-balancer
* VIP's persistence

I modified EnumAttr to allow None for the mapping value to make users
unsert VIP's persistence attribute.

Signed-off-by: Taku Fukushima <48ddf7139da17a0278f1f27e611ac6b9304f94b2@midokura.com>

midonet/python-midonetclient,midokura/python-midonetclient,midonet/python-midonetclient,midokura/python-midonetclient