bullseye: Rename misleading dry-run option

The previous `--dry-run` actually means that bullseye will abandon the
target branch in the end. "Dry run" suggests that it won't touch the
tree, so rename this option into `--cleanup-after`.

Change-Id: I5d24386e59bad40e6232200e4822ab51e75ca880
diff --git a/bullseye/bulletin.py b/bullseye/bulletin.py
index a9f3d13..632c1d8 100644
--- a/bullseye/bulletin.py
+++ b/bullseye/bulletin.py
@@ -90,7 +90,7 @@
 
         if not self.is_applied():
             print("[BE]: Not yet applied: {}".format(self.get_description()))
-            if self.args.apply_to_branch is not None:
+            if self.args.apply_to_branch:
                 self.apply()
         else:
             print("[BE]: Already applied: {}".format(self.get_description()))
@@ -162,7 +162,7 @@
             self.rollback()
         else:
             print("[BE]:    SUCCESS: applied.".format(self.filename))
-            if not self.args.skip_amend and not self.args.dry_run:
+            if not self.args.skip_amend and not self.args.cleanup_after:
                 self.amend()
 
     def exec_command(self, command):