fix: intuitive subnet creation

[NikolaMilosa]

Previously to bootstrap a subnet from n nodes where a user would like to specify some (or all) of the nodes would require setting --size to weird numbers.

For example to create a subnet of 7 nodes, where the user knows all of the nodes they want to use they would issue a command like:

./target/debug/dre subnet create --motivation 'testing' --size 0 \
  --dry-run \ 
  --only 647s4-toz5b-tgvod-ojm2t-g7g3m-kn2le-c36yz-doyb3-rh2fo-ushld-dqe  \
  --only vfyx3-xlume-gp5it-lkwiy-tbjoc-l5ole-f7zm4-vdi63-c4k7w-xrekc-rae \ 
  --only g75ol-eu444-h65jf-ddman-edmen-johle-s2bim-a3yn6-gkpat-5c32o-7ae \ 
  --only vbryd-i5wxp-njyct-thu6d-5ydsn-djzwx-ly4ny-r55cg-sr5k3-piuad-cqe \
  --only m5mj2-rta3p-n4zze-flubl-liaay-3tffk-5nl6d-k5ckv-6mdrg-acnrh-rae \
  --only oauho-ofmnf-bq3eu-z47lj-j3fkh-r7odv-zkh4r-lmia2-vhkl6-r6mgh-cqe \
  --only hckfw-kshvv-mzhew-h6isd-5q2wd-lde6w-uipmv-7gbuz-53wqz-cz5kx-oqe

Furthermore the use of --only and --add was exactly the same. Whatever keyword you used they would end up in the same pool of included nodes.

This PR changes this logic to allow the user to:

1. use actual size of the subnet they want
2. provide exact nodes needed to end up in the subnet (via --add parameter) [optional]
3. provide the pool of available nodes from which the (remaining) nodes should be picked [optional - defaults to all of the nodes]