More advanced task selection // negative filters.

[Gu1nness]

It is currently impossible to select all suites but one, or to provide negative filters to select tasks.
Excerpt from my spread.yaml

project: mongo-charms-single-kernel

backends:
  lxd-vm: ...
  github-ci: ...

suites:
  tests/spread/mongodb/lxd/:
    summary: Spread tests for MongoDB VM
  tests/spread/mongodb/microk8s/:
    summary: Spread tests for MongoDB Kubernetes
  tests/spread/mongos/lxd/:
    summary: Spread tests for Mongos VM
  tests/spread/mongos/microk8s/:
    summary: Spread tests for Mongos Kubernetes
...

I usually select with `spread -list "github-ci:"
I now want to add release tests.

Two paths could be opened:

• Add a new suite: it would make it easy to select only release tests, but very hard not to run those tests on a regular workflow
• Add negative filters/tags:
  • With negative filters: I could be able to run spread -list github-ci::!release and spread -list github-ci::...release...
  • With tags: I could be able to select positively/negatively tests: spread -list github-ci: -tag release and `spread -list github-ci: -tag regular

It's always possible to script around, using the manual flag but I think it would be better to have this directly in spread.