Influxdb Entrypoint Typo

Nobody has claimed this yet.

In the [Influxdb entrypoint script](https://github.com/influxdata/influxdata-docker/raw/master/influxdb/2.1/entrypoint.sh) an environmental variable of `DOCKER_INFLUXDB_INIT_USER` is called out rather then `DOCKER_INFLUXDB_INIT_USERNAME`. The script still works as the command `influx user list -n ''` does return the only user during initial setup.

``` go
function set_init_resource_ids () {
DOCKER_INFLUXDB_INIT_USER_ID="$(influx user list -n "${DOCKER_INFLUXDB_INIT_USER}" --hide-headers | cut -f 1)"
DOCKER_INFLUXDB_INIT_ORG_ID="$(influx org list -n "${DOCKER_INFLUXDB_INIT_ORG}" --hide-headers | cut -f 1)"
DOCKER_INFLUXDB_INIT_BUCKET_ID="$(influx bucket list -n "${DOCKER_INFLUXDB_INIT_BUCKET}" --hide-headers | cut -f 1)"
export DOCKER_INFLUXDB_INIT_USER_ID DOCKER_INFLUXDB_INIT_ORG_ID DOCKER_INFLUXDB_INIT_BUCKET_ID
}
```

Open influxdb/2.1/entrypoint.sh and inspect set_init_resource_ids, especially the variable passed to influx user list. Done means the entrypoint consistently uses the documented username variable while preserving the initial setup behavior; verify the surrounding shell script for matching references.

Written by the indexing model from the issue text.