Add a option to use checksum on get_url in tasks Ensure remote images are downloaded

[roumano]

Add a optional new field in volume : checksum, it's will avoid downloading images when it's not need.
Can be use like this :

  volumes:
    - name: 'debian-10.5.0-amd64-netinst.iso'
      image: https://cdimage.debian.org/debian-cd/current/amd64/iso-cd/debian-10.5.0-amd64-netinst.iso
      checksum: sha256:93863e17ac24eeaa347dfb91dddac654f214c189e0379d7c28664a306e0301e7

[jovial]

Seems like a nice addition. Should we add it to the example in the README?

[roumano]

• Yes, i have added a information on the README.
• Also i added the checksum on the copy tasks (not tested as i don't use this but should work too as it's working exactly same as previous tasks)

[markgoddard]

Thanks