Fix an exception when the host is not connected to host-only interface

[legal90]

Similarly to the problem described in #266, Host-Only interfaces could be also not connected to the host machine, so it causes an exception when Vagrant tries to parse its IP:

       stderr: /Users/legal/.rbenv/versions/2.2.4/lib/ruby/gems/2.2.0/bundler/gems/vagrant-a4f45a18a653/lib/vagrant/util/network_ip.rb:35:in `ip_parts': undefined method `split' for nil:NilClass (NoMethodError)
        from /Users/legal/.rbenv/versions/2.2.4/lib/ruby/gems/2.2.0/bundler/gems/vagrant-a4f45a18a653/lib/vagrant/util/network_ip.rb:19:in `network_address'
        from /Users/legal/Documents/vagrant-parallels/lib/vagrant-parallels/action/network.rb:454:in `block in hostonly_find_matching_network'
        from /Users/legal/Documents/vagrant-parallels/lib/vagrant-parallels/action/network.rb:449:in `each'
        from /Users/legal/Documents/vagrant-parallels/lib/vagrant-parallels/action/network.rb:449:in `hostonly_find_matching_network'
        from /Users/legal/Documents/vagrant-parallels/lib/vagrant-parallels/action/network.rb:323:in `hostonly_adapter'
        from /Users/legal/Documents/vagrant-parallels/lib/vagrant-parallels/action/network.rb:92:in `block (3 levels) in call'
        from /Users/legal/.rbenv/versions/2.2.4/lib/ruby/gems/2.2.0/bundler/gems/vagrant-a4f45a18a653/lib/vagrant/environment.rb:561:in `lock'
        from /Users/legal/Documents/vagrant-parallels/lib/vagrant-parallels/action/network.rb:91:in `block (2 levels) in call'
        from /Users/legal/Documents/vagrant-parallels/lib/vagrant-parallels/action/network.rb:89:in `synchronize'
        from /Users/legal/Documents/vagrant-parallels/lib/vagrant-parallels/action/network.rb:89:in `block in call'
        from /Users/legal/Documents/vagrant-parallels/lib/vagrant-parallels/action/network.rb:75:in `each'
        from /Users/legal/Documents/vagrant-parallels/lib/vagrant-parallels/action/network.rb:75:in `call'

This PR fixes that error by an additional check.
Also, I've removed fetching DHCP details from driver's method #read_host_only_interfaces, because it is actually not needed for Network action.