install-server: add swift cache partition

Shrink root and partitions holding container databases, a fourth partition will
be raid1 and used to provide ssd block caching via bcache.

Root shrinks from 50GB to 30GB, our current utilization is ~27% (i.e. 15G).

Container database partition changes from 90G min to 60G min (120G max),
rationale being that some machines have 256G SSD and some 150G SSD, yet the
same weight in swift. This should accomodate growth on larger SSD while
allowing decent caching on small SSD. Container partition utilization is on
average ~20G (was ~16G beginning of June) so it isn't expected to grow fast
(we'll likely run out of storage space first anyway).

Another option would be to raid1 the two SSDs and switch to lvm to host root,
swap and the cache. That however seems overkill, the bits we really care about
are the partitions holding container databases (sdn3/sdm3) and those will need
to live off raid/lvm anyway.

Note that if caching turns out later to be ineffective, we can drop the fourth
partition in place and grow sdn3/sdm3 back.

Change-Id: Ifc4ff408611868c405d64e605b040072494f7de6