Don't create instance if name is already created

[dpetzel]

This should address #67. The introduction of config_server_name was necessary to avoid these robocop offenses (which cropped up after adding the new conditional in create)

lib/kitchen/driver/openstack.rb:76:7: C: Assignment Branch Condition size for create is too high. [42.8/41]
      def create(state)
      ^^^
lib/kitchen/driver/openstack.rb:76:7: C: Cyclomatic complexity for create is too high. [9/8]
      def create(state)
      ^^^
lib/kitchen/driver/openstack.rb:76:7: C: Perceived complexity for create is too high. [11/10]
      def create(state)
      ^^^

I opted to shift that little bit of logic into its own function to avoid these violations.

[jjasghar]

Nice! This is great. Thank you. I'll test it out today/tomorrow and wait for at least another 👍 from someone else.