Nomis: DSOS-2318: add textfile monitoring role

[drobinson-moj]

Added a separate role to handle nomis batch alerting (textfile monitoring)

Also couple of other things sneaked in:

• Added a new type "exitcode" to more accurately reflect value of other metrics, e.g. 0=good, anything else error code
• Simplified metric paths, e.g. all service checks are under the same metric name - we use dimension instance_type to differentiate between this. This allows us to use a single alarm to cover all services.

Tested on test-oem-a. I have separate ticket to roll out to all servers and adjust the Alarm definitions where appropriate.