Bundle YAML files should use YAML key/value , syntax rather than implement a mini-language in value strings

We should ensure that our YAML files are amenable to syntax highlighting and other forms of static analysis. This will minimise the likelihood that typos will propagate into deployments. For example, bundles recycle the command-line constraints syntax for the application constraints key:

The (currently valid) snippet:

  applications:
    <app>:
     constraints: "mem=1G arch=amd64"

Would ideally be expressed as:

  applications:
    <app>:
      constraints:
        arch: amd64
        mem: 1G

This also appears in kubernetes bundles. The placement directive is a set of key/value pairs wedged into a string.

The placement directive here:

  applications:
    <app>:
      placement: "a=b c=d"

Could be:

  applications:
    <app>:
      placement:
        a: b
        c: d

This would not need to be a breaking change. Juju could inspect the type of the value. When it's presented as a string, interpret using the current behaviour.