Isolate external messages to HookContext in runner

We were referencing `@context` repeatedly in order to send it the
`hook_type_name` and `hook_script_name` messages. Isolate these to
helper methods to emphasize the dependency.

Change-Id: If984ee29f7a14ec78ebc32386dda972126806d8d
Reviewed-on: http://gerrit.causes.com/37379
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>