This adds constraints for asserting that a given attribute or value
exists, or does not exist. This acts as a companion to =, or !=
operators, e.g:
```hcl
constraint {
attribute = "${attrs.type}"
operator = "!="
value = "database"
}
constraint {
attribute = "${attrs.type}"
operator = "is_set"
}
```
This commit allows the ConstraintChecker to test values that do not exist.
This is useful when wanting to _exclude_ given nodes from executing a
job, for example, if you wanted to give canary nodes an attribute, and
not run critical services on them, you may specify something like the
below, but not want to tag all other nodes with the inverse.
```hcl
constraint {
attribute = "${node.attr.canary}
operator = "!="
value = "1"
}
```
This also requires all constraint checkers to allow for nil target
values, as they will no longer be short circuited by resolving a target.
This PR enhances the distinct_property constraint such that a limit can
be specified in the RTarget/value parameter. This allows constraints
such as:
```
constraint {
distinct_property = "${meta.rack}"
value = "2"
}
```
This restricts any given rack from running more than 2 allocations from
the task group.
Fixes https://github.com/hashicorp/nomad/issues/1146
This brings test code and mocks up to date with the fingerprinter. This was a slightly larger change than I anticipated, but I think it's good for two reasons:
1. More semanitcally correct. `os.name` is something like "Windows 10 Pro" or "Ubuntu", while `kernel.name` is "windows" or "linux". `os.version` and `kernel.version` match these semantics.
2. `kernel.name` is much easier to grep for than `os`, which is helpful because oracle can't help us with strings.
