Properties Context

Move all job properties (environmentVariables, throttleConcurrentBuilds, blockOn and priority) to a new properties context.

```groovy
job {
    ...
    properties {
        environmentVariables(...)
        throttleConcurrentBuilds(...)
        blockOn(...)
        priority(...)
    }
}
```

The old methods will stay for the next few releases to retain compatibility, but are marked as deprecated. When running a seed job, deprectation warnings are logged.

```
Warning: environmentVariables is deprecated (job.groovy, line 74)
Warning: throttleConcurrentBuilds is deprecated (DslHelpers.groovy, line 42)
```

This is being done to be able to implement a DSL extension point for the Promoted Builds Plugin, which needs to add a new job property.