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/vault.py

@@ -70,7 +70,8 @@ class VaultCLI(CLI):
         elif self.action == "rekey":
             self.parser.set_usage("usage: %prog rekey [options] file_name")
 
-        self.options, self.args = self.parser.parse_args(self.args[1:])
+        super(VaultCLI, self).parse()
+
         display.verbosity = self.options.verbosity
 
         can_output = ['encrypt', 'decrypt']