Add standalone network plugin

Manila administrator may want to be able to specify network configuration details directly in the manila.conf file
So that he can run Manila with share drivers without need to have the rest of Openstack like Neutron or Nova.

Acceptance Criteria:
1) A "simple" network plugin should be created, which does not depend on anything outside manila
2) The plugin should allow administrators to specify the details of an existing network to which a storage controller is connected
3) The plugin should implement the methods of required of a network plugin, such as provisioning IP addresses
4) This will require some form of persistence to remember which IP addresses have been used and which are available

Suggestions for config option names and example values:
subnet_address=192.168.50.0
subnet_mask=255.255.255.0
subnet_gateway=192.168.50.1
subnet_range=192.168.50.100-192.168.50.199