Merge pull request #174 from tfukushima/mn-2302-v1.4

Change few attributes to be unset
Revert "Change few attributes to be unset"

This reverts commit 38dd1652638d1e3c00d2af17322adcd80afe6479,
which has a syntax error, causing all the Neutron 3rd party CI
jobs to fail.
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>