Increase Vagrant VM memory from 2048 to 4096 MB

Vagrant VM memory on ubuntu xenial uses more memory, so it starts
swapping and gets stuck when running the tests. 4096 MB should be
enough to avoid this.

Change-Id: Iad38f57dbbc7d675f953ec2a76e0bf5e2d9b5b39

Vagrantfile

@@ -1,3 +1,11 @@
+# Note:
+# This file is maintained in the openstack-ansible-tests repository.
+# https://git.openstack.org/cgit/openstack/openstack-ansible-tests/tree/Vagrantfile
+#
+# If you need to perform any change on it, you should modify the central file,
+# then, an OpenStack CI job will propagate your changes to every OSA repository
+# since every repo uses the same Vagrantfile
+
 # Verify whether required plugins are installed.
 required_plugins = [ "vagrant-disksize" ]
 required_plugins.each do |plugin|
@@ -8,7 +16,7 @@ end
 
 Vagrant.configure(2) do |config|
   config.vm.provider "virtualbox" do |v|
-    v.memory = 2048
+    v.memory = 4096
     v.cpus = 2
   end