Remove "-y" option for package install command.

According to "code conventions" [1], do not use "-y" option.
Instead, use apt-get install package, yum install package, or zypper install package.

[1] https://docs.openstack.org/doc-contrib-guide/writing-style/code-conventions

Change-Id: I97549b78b95cce9ba660b5b8e360d2ee5e6bc2a6

README.rst

@@ -71,7 +71,7 @@ Test
 
     $ ``sudo apt-get install npm``
     $ ``curl -sL https://deb.nodesource.com/setup_4.x | sudo -E bash -``
-    $ ``sudo apt-get install -y nodejs``
+    $ ``sudo apt-get install nodejs``
 
   1. ``npm install`` (to create virtual environment and install all dependencies in package.json)
   2. ``npm run lint`` for eslint