Defer loading cmd2

We were importing cmd2 purely so we could do some exception
transformation. However, this is only needed if we're in interactive
mode. Avoid both the import of cmd2 and the transformation of the
exceptions unless this is the case. This speeds up import time by ~30%
for the demoapp on my machine (~160mS after compared to ~210mS before)

Change-Id: I2356dc9803b4d0eef3528c6d057207509932e6b2
Signed-off-by: Stephen Finucane <sfinucan@redhat.com>

cliff/app.py

@@ -20,8 +20,6 @@ import logging.handlers
 import os
 import sys
 
-import cmd2
-
 from cliff import _argparse
 from . import complete
 from . import help
@@ -403,7 +401,12 @@ class App(object):
             try:
                 parsed_args = cmd_parser.parse_args(sub_argv)
             except SystemExit as ex:
-                raise cmd2.exceptions.Cmd2ArgparseError from ex
+                if self.interactive_mode:
+                    # Defer importing cmd2 as it is a slow import
+                    import cmd2
+                    raise cmd2.exceptions.Cmd2ArgparseError from ex
+                else:
+                    raise ex
             result = cmd.run(parsed_args)
         except BrokenPipeError as err1:
             result = _SIGPIPE_EXIT