app: work-around abbrev

cliff does parsing in two phases: it parses the argv options with
parse_known_args(), then find the command, then reparses the command
line again with the parser having being enhanced by the command found.

The problem is that by default, argparse uses prefix matching
https://docs.python.org/3/library/argparse.html#prefix-matching and this
is not possible to disable before Python 3.5. That means that:

$ myapp --endpoint foobar mycommand --start 1 --end 0

is parsed as as namespace with endpoint=0 because --end is read as an
abbrev for --endpoint. This is the case because --end is not registered
until we recognize "mycommand" and register its parsing.

This patches fixes that.

Change-Id: I21f8d9ba580349d2a8fad36bb265033ef64b1278
Closes-Bug: #1503618

cliff/app.py

@@ -1,7 +1,6 @@
 """Application base class.
 """
 
-import argparse
 import codecs
 import inspect
 import locale
@@ -10,6 +9,7 @@ import logging.handlers
 import os
 import sys
 
+from cliff import argparse
 from .complete import CompleteCommand
 from .help import HelpAction, HelpCommand
 from .utils import damerau_levenshtein, COST