pkg-export-helm: Don't consume self
pkg-export-helm: Don't consume self

Chart::generate_manifest_template() should not be consuming self. Instead
put the ManifestJson into an Option so it can be consumed without the
containing struct also be consumed with it.

This is not really an issue yet but in a following patch, we'll need to
call more methods on self after generate_manifest_template() is called.

Signed-off-by: Zeeshan Ali <f7a8e49325fe4a238df42df4f7e30abcfdad87c6@kinvolk.io>

georgemarshall/habitat,nathenharvey/habitat,rsertelon/habitat,nathenharvey/habitat,rsertelon/habitat,nathenharvey/habitat,habitat-sh/habitat,georgemarshall/habitat,rsertelon/habitat,georgemarshall/habitat,nathenharvey/habitat,habitat-sh/habitat,habitat-sh/habitat,rsertelon/habitat,nathenharvey/habitat,habitat-sh/habitat,georgemarshall/habitat,habitat-sh/habitat,rsertelon/habitat,habitat-sh/habitat,habitat-sh/habitat,georgemarshall/habitat,rsertelon/habitat,georgemarshall/habitat,georgemarshall/habitat,habitat-sh/habitat,rsertelon/habitat,georgemarshall/habitat,nathenharvey/habitat