Usage String has Positionals after Optionals

print positionals before optionals in the usage string to help
users avoid argparse nargs='*' problems.

ex: 'trove create --databases <db_name> <name> <flavor_id>'
fails with 'error: too few arguments', but this succeeds:
'trove create <name> <flavor_id> --databases <db_name>'

Change-Id: I400c9f0ca5580bd287c5f3bcf935352e88071bc8
Closes-Bug: #1243981
Merge "Usage String has Positionals after Optionals"