Refactor parsing of CLI args so that we can modify them in the base class

Implement tag and skip_tag handling in the CLI() class.  Change tag and
skip_tag command line options to be accepted multiple times on the CLI
and add them together rather than overwrite.

* Make it configurable whether to merge or overwrite multiple --tags arguments
* Make the base CLI class an abstractbaseclass so we can implement
  functionality in parse() but still make subclasses implement it.
* Deprecate the overwrite feature of --tags with a message that the
  default will change in 2.4 and go away in 2.5.

* Add documentation for merge_multiple_cli_flags
* Fix galaxy search so its tags argument does not conflict with generic tags
* Unit tests and more integration tests for tags

lib/ansible/cli/playbook.py

@@ -75,10 +75,8 @@ class PlaybookCLI(CLI):
         parser.add_option('--start-at-task', dest='start_at_task',
             help="start the playbook at the task matching this name")
 
-        self.options, self.args = parser.parse_args(self.args[1:])
-
-
         self.parser = parser
+        super(PlaybookCLI, self).parse()
 
         if len(self.args) == 0:
             raise AnsibleOptionsError("You must specify a playbook file to run")