beta: define /a on bastion

When creating /a/common symlink, we need a parent /a directory or we end
up having the puppet error:

Change-Id: Ie272869c83a9ef2d6f10456f8febf2700ca33fd8
err: /Stage[main]/Misc::Deployment::Vars/File[/a/common]/ensure:
  change from absent to link failed:
  Could not set 'link on ensure: No such file or directory - /a