* core: Add support for marking outputs as sensitive
This commit allows an output to be marked "sensitive", in which case the
value is redacted in the post-refresh and post-apply list of outputs.
For example, the configuration:
```
variable "input" {
default = "Hello world"
}
output "notsensitive" {
value = "${var.input}"
}
output "sensitive" {
sensitive = true
value = "${var.input}"
}
```
Would result in the output:
```
terraform apply
Apply complete! Resources: 0 added, 0 changed, 0 destroyed.
Outputs:
notsensitive = Hello world
sensitive = <sensitive>
```
The `terraform output` command continues to display the value as before.
Limitations: Note that sensitivity is not tracked internally, so if the
output is interpolated in another module into a resource, the value will
be displayed. The value is still present in the state.
Add `-target=resource` flag to core operations, allowing users to
target specific resources in their infrastructure. When `-target` is
used, the operation will only apply to that resource and its
dependencies.
The calculated dependencies are different depending on whether we're
running a normal operation or a `terraform destroy`.
Generally, "dependencies" refers to ancestors: resources falling
_before_ the target in the graph, because their changes are required to
accurately act on the target.
For destroys, "dependencies" are descendents: those resources which fall
_after_ the target. These resources depend on our target, which is going
to be destroyed, so they should also be destroyed.
James Nugent