dashboard: Allow override of neutron_options

When using specific neutron driver like cisco n1kv we need to have cisco
profile enable in horizon.
This patch allow to override neutron_options in horizon class in order to
enable cisco profile.
It's also useful when we need to activate vpnaas or fwaas in the dashboard.

https://blueprints.launchpad.net/horizon/+spec/horizon-cisco-n1k

Signed-off-by: Dimitri Savineau <dimitri.savineau@enovance.com>
Merge pull request #703 from dsavineau/horizon-cisco-plugin

dashboard: Allow override of neutron_options