cdh plugin yum install option "-y" missing

when I install cdh using sahara with CentOS image,
it occurs timeout exception because of missing option "-y".

Change-Id: Iee9a5d6ab7868fc426c6a82bc7a45228680f8cc6
Closes-Bug: #1566278

sahara/plugins/cdh/commands.py

@@ -67,7 +67,7 @@ def install_packages(remote, packages, timeout=1800):
     if distrib == 'ubuntu':
         cmd = 'RUNLEVEL=1 apt-get install -y %s'
     elif distrib == 'centos':
-        cmd = 'yum install %s'
+        cmd = 'yum install -y %s'
     else:
         raise ex.HadoopProvisionError(
             _("OS on image is not supported by CDH plugin"))