hosted-engine: Update usage info and handle sub-command help

The hosted-engine command uses sub-commands and the current arg parsing
leads to some odd and confusing side effects.  Update the usage message
to make the invocation a bit clearer.  Handle sub-command specific help
(<command> --help) explicitly in each sub-command function.

Signed-off-by: Adam Litke <alitke@redhat.com>
Change-Id: I7323022994b0225b0b319d0b402c0fd755b8da78
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1038626