Added check to prevent adding argument without help string
Added check to prevent adding argument without help string

Added addition check in add_argument() to ensure it has help
message is always present per Amrith's suggestion in this review:
https://review.openstack.org/#/c/261425/

Change-Id: Ic6486570d4beb138fdcd0c4e700ccc8d734d3e5e
Closes-Bug: #1530195

openstack/python-troveclient,Tesora/tesora-python-troveclient,Tesora/tesora-python-troveclient,openstack/python-troveclient