Add peadm::backup plan

[davidsandilands]

A plan that backs up user data, I have tested over the three standard architectures varying what was being backed up and where.

Note it will fail if the backup directory doesn't exist and database backups will fail if pe-postgres doesn't have access to the directory

[reidmv]

LGTM! 👍

I will say, an improvement we can probably make would be to use the peadm::get_peadm_config task to automatically calculate values for replica host, compiler hosts, postgresql host(s), etc. That way, we could run the backup task with just the primary_host parameter as the only required input.

[reidmv]

I'm going to squash and merge for now. Let's consider peadm::get_peadm_config for an easy incremental improvement though.