RBAC : cbbackupmgr fails with authentication error when trying to backup a cluster with a user having data backup role

cbbackupmgr fails when trying to backup a cluster that has only data as a user that has the data_backup role.

Steps :
1. Create a bucket and have some docs in it. No views, no indexes.
2. Create a user backupuser and grant the data_backup role to this user.
3. Run cbbackupmgr as below :
./cbbackupmgr backup -c 10.111.151.101 -u backupuser -p password -a /tmp/backup -r backup

It fails with the following error :
Error backing up cluster: authentication error

The backup log has the following:

2017-07-05T08:58:14.105+00:00 (Plan) Transfering bucket configuration for default to default
2017-07-05T08:58:14.108+00:00 (Rest) GET http://10.111.151.101:8091/pools/default/buckets 200
2017-07-05T08:58:14.109+00:00 (Plan) Transfering views definitions for default to default
2017-07-05T08:58:14.112+00:00 (Rest) GET http://10.111.151.101:8091/pools/default/buckets 200
2017-07-05T08:58:14.113+00:00 (Rest) GET http://10.111.151.101:8091/pools/default/nodeServices 200
2017-07-05T08:58:14.114+00:00 (Rest) GET http://10.111.151.101:8091/pools/default/buckets/default/ddocs 200
2017-07-05T08:58:14.114+00:00 (Plan) Transfering GSI index definitions for default to default
2017-07-05T08:58:14.115+00:00 (Rest) GET http://10.111.151.101:8091/pools/default/nodeServices 200
2017-07-05T08:58:14.125+00:00 (Rest) GET http://10.111.151.101:9102/getIndexMetadata?bucket=default 200
2017-07-05T08:58:14.125+00:00 (Plan) Transfering full text index definitions for default to default
2017-07-05T08:58:14.125+00:00 (Rest) GET http://10.111.151.101:8091/pools/default/nodeServices 200
2017-07-05T08:58:14.126+00:00 (Plan) Deciding which key value data to transfer for default
2017-07-05T08:58:14.126+00:00 (Rest) GET http://10.111.151.101:8091/pools/default/nodeServices 200
2017-07-05T08:58:14.129+00:00 (Rest) GET http://10.111.151.101:8091/pools/default/buckets 200
2017-07-05T08:58:14.130+00:00 ERRO: authentication error – plan.(*dataRange).execute() at range.go:49
2017-07-05T08:58:14.130+00:00 (Plan) Transfer plan failed due to error authentication error
2017-07-05T08:58:14.130+00:00 (Cmd) Error backing up cluster: authentication error