Ensure we add a new line when appending to rc.local

When we add content to rc.local, if the file already
exists then we need to make sure we add the content
after a new line explicitly

Fixes LP #1089668

Change-Id: I35be1496703b302f732363fa76ce832505eed599
(cherry picked from commit f393a51)

nova/virt/disk/api.py

@@ -415,11 +415,14 @@ def _setup_selinux_for_keys(fs):
     # and so to append there you'd need something like:
     #  utils.execute('sed', '-i', '${/^exit 0$/d}' rclocal, run_as_root=True)
     restorecon = [
-        '#!/bin/sh\n',
+        '\n',
         '# Added by Nova to ensure injected ssh keys have the right context\n',
         'restorecon -RF /root/.ssh/ 2>/dev/null || :\n',
     ]
 
+    if not fs.has_file(rclocal):
+        restorecon.insert(0, '#!/bin/sh')
+
     rclocal_rel = os.path.relpath(rclocal, fs)
     _inject_file_into_fs(fs, rclocal_rel, ''.join(restorecon), append=True)
     utils.execute('chmod', 'a+x', rclocal, run_as_root=True)