Add support for static ip addresses

This PR adds support for using static IP addresses in OpenStack. This allows the use of subnets other than the default under a Network.

The original functionality is unchanged if a network is specified by name the name is used as it always was:

    config.vm.provider :openstack do |os|
        ...
        os.networks           = ["MyNetwork"]
        ...
    end

However you also have the option of specifying a hash containing the network name and an optional IPv4 address which is allocated to the machine at boot:

    config.vm.provider :openstack do |os|
        ...
        os.networks           = [ {:name => "MyNetwork", :v4_fixed_ip => "192.168.30.100"} ]
        ...
    end