CP-2190 for PR-1221: host alerts for memory-use and sr-throughput

For now the per-sr throughput alert is configured with a name of the form `sr_io_throughput_total_[0-9a-f]{8}` where the hex at the end is the start of the SR uuid.

Alternatively we could have a simple name like `sr_io_throughput_total` and add a new sub-node to the config variable, something like
`<subsidiary_identifier value="8e41ab3f" />`