simple networks support

You can specify either a network UUID or network name.
Missing or misspelled networks will be ignored.
When provider.networks is not given, vagrant-tiktalik falls back
to default behavior of choosing a first available public network.
```ruby
config.vm.provider :tiktalik do |provider, override|
  provider.networks = [
    [network name],
    [network uuid1],
    [network uuid2],
  ]
end
```