finalize-staged: Bump timeout to 5 minutes

finalize-staged: Bump timeout to 5 minutes

See https://github.com/projectatomic/rpm-ostree/issues/1568

Basically for people on e.g. rotational media, the default 90
second timeout can be too small.

We're in a tough situation here, because delaying shutdown
can be problematic too if the user is trying to shut down their
laptop to put in a backpack, etc.

There's potential optimizations here to make; I think we
could pre-copy the kernel/initramfs for example.

I suspect for some people the grub2 os-prober is a factor here too,
if that tries to e.g. inspect attached USB rotational hard drives.
But hopefully we'll get rid of that soon.

Closes: #1755
Approved by: jlebon