opal_wrapper: consolidate extra variable name

We don't need to have two variables: one named exec_command and one
named exec_cmd.  Consolidate down to just a single variable
(exec_command), and in doing so, fix a compiler warning that
exec_command may not have been initialized (due to confusion between
exec_command and exec_cmd).

Also remove some #if 0/dead code.

Signed-off-by: Jeff Squyres <jsquyres@cisco.com>