Allow setting runroot_method per Pungi phase.
The `runroot_method` now accepts `dict` value with phase name as a key
and runroot method as a value. For backward compatibility, the `str`
value is still supported.
The new `global_runroot_method` option has been added which defines
the runroot method in case it is not set in `dict` in the `runroot_method`.
This commit allows running `createiso` phase locally while keeping the other
phases in Koji.
Signed-off-by: Jan Kaluza <email@example.com>