fix removal of '#cloud-boothook' inside a boothook

The goal was to remove '#cloud-boothook' from a part if the part
started that way.  This was to allow user data of
  #cloud-boothook
  #!/usr/bin/perl
  ...

to be handled correctly.  That had 2 bugs
1.) the prefix string was wrong
2.) was checking for '\r' in the wrong location

cloudinit/__init__.py

@@ -421,11 +421,11 @@ class CloudInit:
         if ctype == "__begin__": return
 
         filename=filename.replace(os.sep,'_')
-        prefix="#cloud-boothooks"
+        prefix="#cloud-boothook"
         dos=False
         start = 0
         if payload.startswith(prefix):
-            start = len(prefix)+1
+            start = len(prefix)
             if payload[start] == '\r':
                 start=start+1
                 dos = True