Add official mechanism for chaining roles together

[sgallagher]

Some roles will require another role to be present on the system (such as nulecule-based roles requiring the kubernetes node role). We should implement this as a basic function of rolekit so that role developers do not need to hack it in.

[sgallagher]

Scope is reduced to listing dependencies and ensuring that they are present before allowing a role that depends on them to be deployed and preventing decommissioning if anything still depends on it.

[sgallagher]

Closing as wontfix. This can be solved more simply by playing with 'After=' in the systemd unit files.