Externalize the ntp and iptables cookbooks

We now configure ntp and iptables through attributes and run them
through the chef-solo runlist.

We keep them in the metadata file so that berks packages them for us.
Make NTP and iptables management optional
Re-create run list everytime the installer runs

This ensures that options like --no-ntp are taken into account, and
aren't discarded based on an existing solo.json
Conditonally ask for SSH key based on git repo URL

It's not useful to ask for a private key when using a HTTP URL, and it's
very confusing. Only ask for it when it looks like it might be needed.
Refactor install script prompts

+ Add the option to not use the host's IP as the endpoint.
+ Reverse the repo and revision options to something more sensible.
+ Update the text in general to make it clearer
Strip whitespace by default in wrapper prompts

This ensures that e.g. you don't end up with trailing space in your
endpoint URL.
Prepare the Scalr 5 config for Scalrpy refactor
Release: 6.4.0
Merge pull request #60 from Scalr/release-6.4.0

Release 6.4.0