feat: nodeshift login with cli

[lholmquist]

This PR adds the login command. It is meant to be run first with either a username/password or a valid token and the api server of the cluster

something like this: nodeshift login --token=12345 --server=https....... then run nodeshift deploy. While you can specify all those argument flags with the deploy command, this cleans things up a bit

The new login command will write the authtoken and the api server to a login.json file that lives in PROJECT_LOCATION/tmp/nodeshift/config/login.json. this tmp directory is also used in the build and deploy steps, and should be gitignored.

CLI Usage - Login:

$ nodeshift login --username=developer --password=password --server=https://api.server

or 

$ nodeshift login --token=12345 --server=https://api.server

CLI Usage - Logout

$ nodeshift logout

API usage using async/await would look something like this:

const nodeshift = require('nodeshift');

const options = {
  username: 'kubeadmin',
  password: '...',
  server: '...',
  insecure: true
};

(async () => {
  await nodeshift.login(options);
  await nodeshift.deploy();
  await nodeshift.logout();
})();

[coveralls]

Coverage decreased (-98.3%) to 0.0% when pulling 999eaca on lholmquist:NODE-899-nodeshift-login into 509f0a5 on nodeshift:master.