ostree admin: Fix return value from 'ostree admin [instutil]'

'ostree admin' and 'ostree admin instuil' with no arguments were meant to fail,
but the logic was wrong; add an assertion on the return value from all ostree
commands to catch similar problems in the future.

https://bugzilla.gnome.org/show_bug.cgi?id=737194
Pass --help to the most nested subcommand

'ostree admin <x> --help' and 'ostree admin instutil <x> --help' should
give help on the deepest subcommand, not on 'ostree admin'.

https://bugzilla.gnome.org/show_bug.cgi?id=737194
Fix help output for nested subcommands

Only 'ostree admin' was appearing in the help message for commands
nested within ostree admin.

https://bugzilla.gnome.org/show_bug.cgi?id=737194
ostree admin instutil: make --help work for subcommands

Even though the subcommands don't take arguments, use a GOptionContext
so that --help works as expected.

https://bugzilla.gnome.org/show_bug.cgi?id=737194
--help should always go to stdout

The standard convention is that the  output of --help should go to standard
output (so that it can be piped to a pager and searched.) See, e.g., the
GNU coding standards.

https://bugzilla.gnome.org/show_bug.cgi?id=737194
Add test for the behavior of --help

Recursive over ostree and all subcommands, and check that --help
is supported, properly outputs to standard out, and exits
with a 0 exit status. Check that for commands with subcommands,
they produce the help output to standard error when run with no arguments.

https://bugzilla.gnome.org/show_bug.cgi?id=737194