Upgrade conda-store v0.3.10 and simplify specification of image

[HarshCasper]

Fixes #1126

[danlester]

@HarshCasper Oh no, the image quansight/conda-store-server:v0.3.10 still appears in two places!

Sorry, I didn't realise that both conda-store-server and conda-store-worker use the same image... So I might have pointed you in the wrong direction.

Can you find another way to do all of this so that, essentially, the string quansight/conda-store-server:v0.3.10 (or related objects) appears only once?

There are many approaches you could take.

(1) Maybe reinstate all the Terraform variables but with no defaults in the Terraform variable definitions, and then pass through quansight/conda-store-server:v0.3.10 from input_vars.py as we did before.

(2) Or keep the string removed from input_vars.py and reinstate the Terraform variables plus a default only at one level - probably in conda-store.tf you would add the default {...} image setting, but make sure you do not also set the default in conda-store/variables.tf.

Or there are other possibilities. Personally I think (2) suits us best at this time - we will easily be able to override the image from Python code in the future if we want to.

In (2) this means you can't use the lowest level module (modules/kubernetes/services/conda-store or whatever as a folder) without explicitly passing through a conda-store-image value as we will do from conda-store.tf. But that's fine - the module is unlikely to be used separately from its surrounding codebase so we don't need to worry about it, and in any case whatever uses it can just pass through the value explicitly.

[danlester]

Changes requested as above.

[danlester]

Closes #1121