cli: make --plugin-path and --omit-{system,home}-plugin-path global opts

cli: make --plugin-path and --omit-{system,home}-plugin-path global opts

This patch makes the `--plugin-path`, `--omit-system-plugin-path`, and
`--omit-home-plugin-path` options global options, in that you must
specify them before the command's name.

For example, this used to work:

    $ babeltrace2 list-plugins --plugin-path=mein-plugins

With this patch it doesn't, as `--plugin-path` must come before the
command name:

    $ babeltrace2 --plugin-path=mein-plugins list-plugins

This change exists because those three options were handled by all five
commands anyway, so there was a lot of code redundancy.

Tests are adapted accordingly. Everything about those three options in
`test_convert_args` is removed as they will never be part of the
`--run-args` output now.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I6169b01c2c3943bc5ba13ff0be68f8f8a1853da2
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1951
Tested-by: jenkins <jenkins@lttng.org>