auto-installer: shorten partition label to fit in 11 characters for FAT

auto-installer: shorten partition label to fit in 11 characters for FAT

(v)FAT is still the most popular FS on removable medias like USB pen
drives, so we need to cope with its legacy..

So, rename the label that we look for again to PROXMOX-AIS, as that
still should allow admins to better recognize what this might be for
by having the "Proxmox" context and the rest is just hard to get
sensible meaning in, so do not bother to invent elaborate explanations
of why AIS is a good choice, it isn't but it's about the best thing
that one can do in these constraints.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>