admin: Add new run-triggers command

admin: Add new run-triggers command

In some cases we want the ability to run triggers independently of
checking out a tree.  For example, due to kernel limitations which
impact the gnome-ostree build system, we may need to run triggers on
first boot via systemd.

Secondarily, if the user installs a system extension which adds a new
shared library to /usr/lib for example, the system will need to run
the triggers again.

Also, I think I want to take triggers out of the core and put them in
ostree admin anyways.