Add .old dir support for source services

Some services expect "old" service files (that is, files from a
previous service run) to be present in an ".old" dir. Hence, osc
should support that.
Instead of removing all files from a previous service run, move them
to the ".old" dir, run the services, and, finally, remove the ".old"
dir.
Unfortunately, the location of the ".old" dir is hardcoded in the
specific services. That is, we have to be careful if an ".old" dir
exists (in this case, we error out).

Based on [1].

[1] #846

osc/core.py

@@ -410,16 +410,27 @@ def addRecompressTar(self, serviceinfo_node):
         return r
 
     def execute(self, dir, callmode = None, singleservice = None, verbose = None):
+        old_dir = os.path.join(dir, '.old')
+        if os.path.exists(old_dir) or os.path.islink(old_dir):
+            msg = '"%s" exists, please remove it' % old_dir
+            raise oscerr.OscIOError(None, msg)
+        try:
+            os.mkdir(old_dir)
+            return self._execute(dir, old_dir, callmode, singleservice,
+                                 verbose)
+        finally:
+            if os.path.exists(old_dir):
+                shutil.rmtree(old_dir)
+
+    def _execute(self, dir, old_dir, callmode=None, singleservice=None,
+                 verbose=None):
         import tempfile
 
         # cleanup existing generated files
         for filename in os.listdir(dir):
             if filename.startswith('_service:') or filename.startswith('_service_'):
-                ent = os.path.join(dir, filename)
-                if os.path.isdir(ent):
-                    shutil.rmtree(ent)
-                else:
-                    os.unlink(ent)
+                os.rename(os.path.join(dir, filename),
+                          os.path.join(old_dir, filename))
 
         allservices = self.services or []
         service_names = [s['name'] for s in allservices]