fix how we automatically set cluster.enable_longhorn

We are setting s-c-c cluster.enable_longhorn based on whether the longhorn unit is enabled or not.

This currently does not work how we want it to work for bootstrap phase, because during bootstrap phase the longhorn unit isn't enabled, even when enabled in environment values because it's not part of the units we need for bootstrap that are selected by units_override_enabled.

As a result, the s-c-c release that defines the mgmt cluster receives enable_longhorn: false during bootstrap, which results in the longhorn steps (labels, annotations, disk mounts) to not be defined.

This MR solves this by also setting enable_longhorn to true if the unit is enabled in values.

Solving this is needed to progress !1007 (merged)