Enable remote execution of repmgr via a different system user

[ibarwick]

Currenly, when executing repmgr on a remote note, repmgr assumes the SSH user it's connecting as is the postgres user. This however makes it impractical to use a different SSH user if the operation needs to read the data directory, e.g. node status --is-shutdown-cleanly .

To resolve this, add some way of optionally configuring the remote invocation to use sudo.