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 <tfukushima@midokura.com>
Merge pull request #173 from tfukushima/mn-2302-master

Change few attributes to be unset