Defer loading cmd2
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 <492121341a95b3c3aab646bed44634f739dd019b@redhat.com>

openstack/cliff,openstack/cliff