Proper handling of requested disk size

[alahiff]

Currently disk size is ignored (!) Need to handle this properly, noting that:

• When determining what flavours are suitable, note that for multi-node MPI jobs the resources are aggregated
• We could request additional volumes for jobs running on a single VM (not currently done). This is more complex, as we would also need to check that we have enough quota to do this.

[alahiff]

Including disk size in choice of flavour implemented here: https://github.com/prominence-eosc/imc/commit/6c1b8bec64ad511bcae3c37250cc841e1406e526

Pass requested disk size through to IMC implemented here: https://github.com/prominence-eosc/prominence/commit/30d79850aec4d1fccf2fb0e98bd18e4833c9ecec#diff-738057f70c2838e25604349699fbfee3

[alahiff]

Disk size is now taken into account (updated OPA policies).