Update posts-via-wp-cli to use login shorthand

[bph]

The current steps

	"landingPage": "/wp-admin/edit.php",
	"steps": [
		{
			"step": "login",
			"username": "admin",
			"password": "password"
		},
		{
			"step": "wp-cli",
			"command": "wp post generate --count=12 --post_type=post --post_date=1999-01-04"
		}
	]

Become

	"landingPage": "/wp-admin/edit.php",
	"login":true
	"steps":[
	{
		"step": "wp-cli",
		"command": "wp post generate --count=12 --post_type=post --post_date=1999-01-04"
	}
	]