key file names should contain stack ID and cluster

[dafydd-t]

PDC agent writes multiple files to the default ~/.ssh directory as part of its setup. When running multiple PDC agents to different stacks from the same machine, this can cause ssh key ovewrites unless the --ssh-key-path flag is used.

We can avoid this behaviour by default by adding the stackID and cluster value to the file names - including known hosts.

So, for stack 123 in region prod-us-east-0, instead of these:

grafana_pdc
grafana_pdc-cert.pub
grafana_pdc.pub
grafana_pdc_hash
grafana_pdc_known_hosts

We should have something like:

grafana_pdc_123_prod-us-east-0
grafana_pdc_123_prod-us-east-0-cert.pub
grafana_pdc_123_prod-us-east-0.pub
grafana_pdc_hash_123_prod-us-east-0
grafana_pdc_known_hosts_123_prod-us-east-0