Fix config help order to be alphabetical

Currently The output of --help is in random order. The only way using public
argparse api to control the order of display of --help is to add the arguments
in the sorted order. So added a caching mechanism to collect all the calls
to add_argument and ensure they are sorted before calling argparse's
add_argument. The only other choice if anyone is interested is documented
here which uses the non-public API

http://stackoverflow.com/questions/12268602/sort-argparse-help-alphabetically

Fixes LP# 1185959

Change-Id: I5f934c85fd516e2d87b3ece40142f16c898b04ce