Add custom ChefSpec matcher.
Update README.md
Do not hash public keys [COOK-4607]

Hashes are different on each ssh-keyscan run, therefore filling up
/etc/ssh/ssh_known hosts
Cache known hosts entries in a data bag
Fix the search string so that it actually finds things (at least on test kitchen)
Switch to chef-zero for testing search
Add test data
Add a bunch of debug logging and check existing data bag contents before storing new cache entries
Move cacher attributes to default attributes file
Make test keys somewhat unique
Add option to get keys from the data bag key cache (rather than search)
Add test scenario for the cacher client
Make the test data more readable
Set up ServerSpec tests
Describe how to use cacher recipe in the readme
Adds basic test to verify matcher
Add kitchen config for DO testing
Update to serverspec 2
Add more test cases