Dynamic custom config generation bug

[zavale]

In a case, that documentation variable added dynamically to custom, by generate.js script, this.customVars remains uninitialized, since generate.js runs after the plugin constructed. That causes an immediate return at if (!(this.customVars && this.customVars.documentation)) return; check.

Reinitializing this.customVarsin beforeDeploy() solves the problem

However, it will require modification in the test

    it('shouldn\'t do anything if there are no custom variables', function () {
      delete this.plugin.customVars;
      this.plugin.beforeDeploy();
      expect(this.serverlessMock.service.getAllFunctions).not.toHaveBeenCalled();
    });