More advanced task selection // negative filters.

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