`--force` was previously available only on `spack concretize`
but not on `spack spec`, `spack solve`, and other commands that
do concretization.
This means you can now preview a force-concretize on an environment
or spec with `spack spec -f` or `spack solve -f`. You can also set
concretization to *always* force in config with:
```yaml
spack:
concretizer:
force: true
```
- [x] make `concretizer:force` a configuration option
- [x] add `--force` to common concretizer arguments
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