Exchange used algorithm in ssh_keygen from dsa to rsa

DSA should not be used anymoe, as ssh-keygen limits its length to 1024,
which shouldn't be considered secure anymore.
Add data_bag support for ssh_keys

Get public keys from data bag instead of providing them as strings
Automatically detects whether ssh_keys is a public key.
If it is not, it will look for an item in the data_bag
Add data_bag documentation to README
Merge branch 'rsa'