Rework udev_settle timeout handling (#544177)

1) centralize the setting of the timeout parameter inside the
udev_settle function. I do realize that the time needed for
initially scanning all disks is much larger, then the time for
scanning a single new partition when we are creating partitions.
But since this is a timeout, and udevadm settle will exit as soon
as the scanning is done, I see no reason not to have a single place
to configure the timeout assuming the worst case scenario (the initial
disk scan). This way we won't have these magic values sprinkled all
over the code.

2) Given that in the past we've had bugs filed
about anaconda taking many minutes to scan hardware on big iron machine
and that our initial udev settle call will scan all disks, I think
that udevadm's default timeout of 1 minutes is too low, so this patch
sets the timeout to 5 minutes.

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