metadata and doc generator optimization and fixes

* Fix ansible-doc traceback when a plugin doesn't parse correctly
* Change extract_metadata ivocation to take either an ast or source
  code.  When given source code, it can find file offsets for the start
  and end of dict.  When given the ast, it is quicker as it doesn't have
  to reparse the source.  Requires changing the call to the function to
  use a keyword arg.
* Fix reading of metadata to find the last occurrence of
  ANSIBLE_METADATA instead of the first.
* Add some more unittests to get closer to complete coverage

hacking/metadata-tool.py

@@ -205,7 +205,8 @@ def write_metadata(filename, new_metadata, version=None, overwrite=False):
         module_data = f.read()
 
     try:
-        current_metadata, start_line, start_col, end_line, end_col, targets = extract_metadata(module_data)
+        current_metadata, start_line, start_col, end_line, end_col, targets = \
+            extract_metadata(module_data=module_data, offsets=True)
     except SyntaxError:
         if filename.endswith('.py'):
             raise
@@ -257,7 +258,7 @@ def return_metadata(plugins):
         if name not in metadata or metadata[name] is not None:
             with open(filename, 'rb') as f:
                 module_data = f.read()
-            metadata[name] = extract_metadata(module_data)[0]
+            metadata[name] = extract_metadata(module_data=module_data, offsets=True)[0]
     return metadata
 
 
@@ -408,7 +409,7 @@ def upgrade_metadata(version=None):
         # For each plugin, read the existing metadata
         with open(filename, 'rb') as f:
             module_data = f.read()
-        metadata = extract_metadata(module_data)[0]
+        metadata = extract_metadata(module_data=module_data, offsets=True)[0]
 
         # If the metadata isn't the requested version, convert it to the new
         # version