base64 encode passwords and simplify .brooklyn_cli structure

base64 encode passwords and simplify .brooklyn_cli structure

Includes support for old style .brooklyn_cli format such that it will
read credentials from the old style and use them, but overwrite with
the new style on next login.

New style looks like:
{
  "credentials": {
    "password": "cGFzc3dvcmQ=",
    "username": "geoff"
  },
  "skipSslChecks": false,
  "target": "http://geoffs-macbook-pro.local:8081"
}