Previously, config information was stored per stack. With this change,
we now allow config values which apply to every stack a program may
target.
When passed without the `-s <stack>` argument, `pulumi config`
operates on the "global" configuration. Stack specific information can
be modified by passing an explicit stack.
Stack specific configuration overwrites global configuration.
Conside the following Pulumi.yaml:
```
name: hello-world
runtime: nodejs
description: a hello world program
config:
hello-world:config:message Hello, from Pulumi
stacks:
production:
config:
hello-world:config:message Hello, from Production
```
This program contains a single configuration value,
"hello-world:config:message" which has the value "Hello, from Pulumi"
when the program is activated into any stack except for "production"
where the value is "Hello, from Production".
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