Improve handling of repmgrd's "promote_command" and "follow_command" parameters

[ibarwick]

Historically, repmgrd (when operating on a streaming replication node) has required provision of the promote_command and follow_command parameters, which should contain either repmgr standby promote/repmgr standby follow, or scripts which at some point execute those commands.

This is becoming a source of confusion, particularly with promote_command as it implies this will be used in other contexts where the local PostgreSQL node will be promoted (e.g. during switchovers); see e.g. #473.

To mitigate this, rename the parameters to repmgrd_promote_command and repmgrd_follow_command respectively, keeping the original parameters for backwards compatibility.

Additionally, investigate whether it makes sense to have repmgrd perform the promote/follow actions directly, if no values supplied for these parameters; this will make basic configuration easier.