Add --assume-yes option to apt-get

During the installtion of python-yaml, apt-get shows a prompt
and it stops devstack-gate if python-yaml is not installed.

Change-Id: Iabe6d1a7dd63149bce9b0473fae2ff994f7f0856

devstack-vm-gate.sh

@@ -69,7 +69,7 @@ function setup_localrc {
         # Install PyYaml for test-matrix.py
         if uses_debs; then
             sudo apt-get update
-            sudo apt-get install python-yaml
+            sudo apt-get --assume-yes install python-yaml
         elif is_fedora; then
             sudo yum install -y PyYAML
         fi