Remove the master option from the charm config to drop support for master/slave replication. Specify the replSet option in the /etc/mongodb.conf file instead of modifying the /etc/init/mongodb.conf file.
description: Replica Set master (optional). Possible values are 'auto' for automatic detection based on install time or 'host:port' to connect to 'host' on 'port' and register as a member.
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master:
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default: "self"
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type: string
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description: Who is the master DB. If not "self", put the Master DB here as "host:port"