Full support for custom timeout in testcloud reboot

[psss]

Currently setting the maximum time for a reboot in the testcloud plugin works only when a custom reboot command is used, for example like this:

tmt-reboot -t 60 -c reboot

It would be nice if we support timeout without the custom command as well:

tmt-reboot -t 60

The problem was revealed by the following test:

/tests/execute/reboot/shorten-timeout

Related code is here:

tmt/tmt/steps/provision/testcloud.py

Lines 1078 to 1083 in 6e4742b

	# Use custom reboot command if provided
	if command:
	return super().reboot(hard=hard, command=command)
	if not self._instance:
	raise tmt.utils.ProvisionError("No instance initialized.")
	self._instance.reboot(soft=not hard)

Ideally testcloud would provide timeout parameter in instance.reboot() so that we can hand over the value.

[psss]

@frantisekz, what do you think?