Add the OS_* env variables to mirror the NOVA_* vars; example:
setting OS_USERNAME will override NOVA_USERNAME in the clients and
tools, but if left unset it defaults to NOVA_USERNAME.
Adds exercises/client-env.sh to test operation of command-line
clients with only the OS_* variables set
Addresses bug 897304, http://wiki.openstack.org/CLIAuth
Change-Id: I72450153541072fe8026a82748cfcd1cf5ed31d8
10670d1bad