Default disk selection is irritating and disk selection is inflexible

[Ian Wells]

The default install disk is sdc. This doesn't work with most machines, but it's chosen because it used to be common for UCSes with various BIOSes.

However this conceals a wider problem, that 'sda' is also not the right choice for our test disks or for some users.

We propose a different default:

1. set the setting to 'first-disk' by default
2. in the preseed's preinstall commands, check for that and find the first usable disk per

    https://github.com/sorenh/python-django-greenfan/blob/master/greenfan/templates/build-node.preseed.tmpl#L5

which goes and looks for the first disk with a size (excluding the UCS virtual devices that sometimes enumerate first).

Run this iff the setting is first-disk and overwrite it on discovery.

In the future if we need other strategies (e.g. 'find me the disk on SCSI bus 2 LUN 0' - useful for mixed driver setups, as device ordering is not guaranteed with mixed drivers) we can add other meaningful values for the disk selection.