Change few attributes to be unsettable

This is the revival of 38dd1, which is reverted due to the typo.

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
unset VIP's persistence attribute.

Signed-off-by: Taku Fukushima <tfukushima@midokura.com>
Merge pull request #176 from tfukushima/mn-2302-v1.4

Change few attributes to be unsettable