Add overwrite option when importing pipelines and ces

[JaimeSeqLabs]

Description

Adds '--overwrite' flag to pipeline and CE import commands.
If the flag is set the resource is deleted before importing the data.

Closes #285

Guidelines for testing

tw compute-envs import --overwrite -n TestCE import_data.json

tw pipelines import --overwrite -n TestPipeline import_data.json

[drpatelh]

Thanks @JaimeSeqLabs ! Would it be possible to extend the scope of this PR to --overwrite any entity in Tower? This is particularly useful in automation scenarios where you want to re-run a script and not have to delete everything individually.

For example, if you search for overwrite in this file you can see it is possible to set it for any entity and not just Pipelines / CEs as requested in the linked issue.
https://github.com/seqeralabs/twkit/blob/661f4e12ecd86fbc54414a26456fbbacff4b773a/examples/yaml/twkit-e2e.yml#L7

Having this functionality in the CLI will mean we can remove the customisation we are using in twkit.

Thanks!

[JaimeSeqLabs]

Hi @drpatelh
Yes, it is possible to support --overwrite for more entities, but each one needs to be deleted and re-created explicitly in code, there is not a common approach to it.
I would prefer creating another issue for supporting the rest of the entities just to keep things tidy.

[JaimeSeqLabs]

See #338 for the aggregated list