Drop nova network support from Horizon

nova-network feature is deprecated in Newton and nova team drops corresponding features from novaclient in 8.x.x recently in Pike. It seems time to drop nova-network support in Horizon.

This work covers:
- delete corresponding API wrapper (security group and floating IP) from openstack_dashboard.api.nova
- delete back-end check in the panel implementations
- delete network abstraction API wrapper (openstack_dashboard.api.network and network_base)

Note that security group association per server is out of scope of this work (as Nova server API still supports security group association when creating a server).

There are several deprecation and drop plans:

(a) Drop horizon support for nova-network in Pike based on nova-network deprecation in Nova Newton release
(b) Deprecate horizon support for nova-network in Pike and say nova-network support MAY work with older novaclient <8.x.x).
    Drop nova-network support in Queen release
(c) Deprecate horizon support for nova-network in Pike and implement horizon-specific python bindings which use older nova API micro-version. Drop nova-network support in Queen or R release.

There may need a discussion in the operator ML as well.

Related discussion in the dev ML.
Mar 2017: http://lists.openstack.org/pipermail/openstack-dev/2017-March/thread.html#114674
Apr 2017: http://lists.openstack.org/pipermail/openstack-dev/2017-April/thread.html#115194