Maintenace and cleanup

[tomzo]

I have marked a few points which could be improved https://github.com/gocd-contrib/go-cookbook/commit/867088c17a587a3d3e01b5e4a275d48c07aa466d Issues I am planning to fix:

• test compatibility with chef 11 and 12
• attempt to make node['go']['version'] = 'stable' by default (#26). Is there an URL that would return what is stable currently?
• refactor and document the backup and restore functionality which apparently works, but has no tests or readme
• add more detailed integration tests (test auto-registration of agents in particular that they are added and enabled)
• add chefspec tests
• extend readme on how to use the cookbook on various platforms
• I'd also like to assume that chefdk is now the primary development toolset
• setup snap-ci to run stylechecks, chefspec and kitchen integration tests in docker (as @ketan already did). If this will not be possible in long-term then I will be running integration tests on openstack locally.

I think it would also be nice to have LWRP for go agents. I will describe in separate issue. I'd also like to split recipes into just installing Go packages and actually managing services. Default would be still to do both. Probably I will write more tests before I add or refactor anything so that I will see if I break anything.

I will not do this all at once, but in time I should cover all of it.