Fix certificate ordering logic

* app certs set by user are always preferred
* If fallback, choose fallback certs. ignore others
* If LE, try to pick LE certs. Otherwise, provider fallback.

Fixes #724

src/cert/fallback.js

@@ -1,22 +0,0 @@
-'use strict';
-
-exports = module.exports = {
-    getCertificate: getCertificate,
-
-    // testing
-    _name: 'fallback'
-};
-
-var assert = require('assert'),
-    debug = require('debug')('box:cert/fallback.js');
-
-function getCertificate(hostname, domain, options, callback) {
-    assert.strictEqual(typeof hostname, 'string');
-    assert.strictEqual(typeof domain, 'string');
-    assert.strictEqual(typeof options, 'object');
-    assert.strictEqual(typeof callback, 'function');
-
-    debug('getCertificate: using fallback certificate', hostname);
-
-    return callback(null, '', '');
-}

src/cert/interface.js

@@ -1,24 +0,0 @@
-'use strict';
-
-// -------------------------------------------
-//  This file just describes the interface
-//
-//  New backends can start from here
-// -------------------------------------------
-
-exports = module.exports = {
-    getCertificate: getCertificate
-};
-
-var assert = require('assert'),
-    BoxError = require('../boxerror.js');
-
-function getCertificate(hostname, domain, options, callback) {
-    assert.strictEqual(typeof hostname, 'string');
-    assert.strictEqual(typeof domain, 'string');
-    assert.strictEqual(typeof options, 'object');
-    assert.strictEqual(typeof callback, 'function');
-
-    return callback(new BoxError(BoxError.NOT_IMPLEMENTED, 'getCertificate is not implemented'));
-}
-