Admin Network support in Generic Driver

Add admin network support in generic driver.

A neutron network should be created to be used as admin network for the generic driver.
The administrator can either configure a provider network or a vxlan network for use with the generic driver. Since the generic driver already uses vxlan by default, this proposal creates a port and a tunnel between the neutron network and the host.

When admin network is configured, every Share server Virtual Machine wil have a network interface in this neutron network, which is accessible by the admin nodes.

Since Service VM currently can only have 2 network interfaces, only the following scenarios are possible:

#1: service network.
#2: service network and tenant network.
#3: service network and admin network.
#4: tenant network and admin network.

For the purpose of Manila CI, the routes are automatically configured in the host. In a production environment, a single route has to be configured in the other nodes to the configured network.

The driver will read a specific set of parameters related to the admin network, such as:
admin_network_id
admin_subnet_id

This proposal also creates a admin export location for the generic driver, since Manila code already supports this feature.