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/adhoc.py

@@ -72,7 +72,7 @@ class AdHocCLI(CLI):
             help="module name to execute (default=%s)" % C.DEFAULT_MODULE_NAME,
             default=C.DEFAULT_MODULE_NAME)
 
-        self.options, self.args = self.parser.parse_args(self.args[1:])
+        super(AdHocCLI, self).parse()
 
         if len(self.args) < 1:
             raise AnsibleOptionsError("Missing target hosts")
@@ -82,8 +82,6 @@ class AdHocCLI(CLI):
         display.verbosity = self.options.verbosity
         self.validate_conflicts(runas_opts=True, vault_opts=True, fork_opts=True)
 
-        return True
-
     def _play_ds(self, pattern, async, poll):
         check_raw = self.options.module_name in ('command', 'win_command', 'shell', 'win_shell', 'script', 'raw')
         return dict(