Provide a way for adding `dependsOn` HRs to `packages`

Currently the packages block allows for deploying an arbitrary package that depends on policy enforcement and conditionally on istio. In some cases an end user might want their chart to depend/wait on a different BB package (ex: monitoring).

As part of the package values we should expose a value such as dependsOn: [] to allow someone to add other dependencies for their package.

Extra fix required - the current namespaces for dependsOn should always be Big Bang, they should not have {{ default "bigbang" $.Values.namespace }} as that just confuses things and allows for errors.