Fixed the subcommand error message for nova shell.
Fixed the subcommand error message for nova shell.

Previously we were giving advice like this:

"Try 'nova volume-show help' for more information."

Bad advice -- that doesn't work. Now when there's a subcommand
specified we make a proper suggestion:

"Try 'nova help volume-show' for more information."

Change-Id: I6ef49f9e4e1b67074f51ab442abd4a196d437b00

metacloud/python-novaclient,mat128/python-novaclient,mat128/python-novaclient,CCI-MOC/python-novaclient,JioCloud/python-novaclient,akash1808/python-novaclient,ntt-sic/python-novaclient,Shihta/python-novaclient,ntt-sic/python-novaclient,CCI-MOC/python-novaclient,guptaankita/python-novaclient,metacloud/python-novaclient,takeshineshiro/python-novaclient,citrix-openstack-build/python-novaclient,JioCloud/python-novaclient,citrix-openstack-build/python-novaclient,guptaankita/python-novaclient,akash1808/python-novaclient,takeshineshiro/python-novaclient,Shihta/python-novaclient