Test multiple images of the same chain/para

[wirednkod]

Following the request of this PR for backward compatibility testing, it would make sense to have a way to generically test different branches in parallel, with firing up 2 different images and being able to pass the secondary_image into one of the relay chain nodes as can be seen below:

add env vars:

export ZOMBIENET_POLKADOT_IMAGE_MASTER=docker.io/paritypr/polkadot-debug:master
export ZOMBIENET_POLKADOT_IMAGE_BRANCH=docker.io/paritypr/polkadot-debug:branchX

and in the configuration of Zombienet:

[relaychain]
default_image = "{{ZOMBIENET_POLKADOT_IMAGE_MASTER}}"
secondary_image = "{{ZOMBIENET_POLKADOT_IMAGE_BRANCH}}" 
chain = "rococo-local"

  [[relaychain.nodes]]
  name = "alice"

  [[relaychain.nodes]]
  name = "charlie"
  image = "{{secondary_image}}"

  [[relaychain.nodes]]
  name = "bob"

cc @pepoviola

[wirednkod]

Apparently this already exists by setting the image to use at node level and that will override the default_image:
e.g

[relaychain]
default_image = "{{ZOMBIENET_INTEGRATION_TEST_IMAGE}}"
chain = "rococo-local"

  [[relaychain.nodes]]
  name = "alice"
  image = "{{ZOMBIENET_INTEGRATION_TEST_IMAGE_SECONDARY}}"
  [[relaychain.nodes]]
  name = "bob"

Closing this issue