Unable to restore backup to existing bucket

__Steps to reproduce:__

1. Create backup `# influx backup -b bucket1 ~/influxdb-v2-export/`
2. Copy files to different server where bucket already exists with data in it
3. Attempt to restore `# influx restore -b bucket1 ~/influxdb-v2-export/`

__Expected behavior:__

Data is restored to existing bucket along side the already existing data in that bucket.

Documentation ([https://docs.influxdata.com/influxdb/v2.0/reference/cli/influx/restore/](https://docs.influxdata.com/influxdb/v2.0/reference/cli/influx/restore/)) provides an example for restoring data to an existing bucket separate from restoring to a new bucket.

__Actual behavior:__

Errors out saying "cannot create bucket".

```
2021-02-15T19:34:58.266935Z info Resources opened {"log_id": "0SLnrsqW000", "path": "influxdb-v2-export/20210215T185834Z.bolt"}
2021-02-15T19:34:58.267266Z info Restoring organization {"log_id": "0SLnrsqW000", "id": "583d63a670ab779c", "name": "org1"}
2021-02-15T19:34:58.268984Z info Restoring bucket {"log_id": "0SLnrsqW000", "id": "04ea7e91e2f1a222", "name": "bucket1"}
Error: Cannot create bucket: bucket with name bucket1 already exists.
See 'influx restore -h' for help
```

__Environment info:__

* System info: Linux 4.18.0-277.el8.x86_64 x86_64
* InfluxDB version: InfluxDB 2.0.3 (git: fe04d346df) build_date: 2020-12-15T01:00:16Z
* Other relevant environment details: Source and destination systems are running identical version of InfluxDB

Start with the `influx restore` CLI command and reproduce the backup and restore steps against an existing bucket. Trace the restore behavior described in the issue and verify that existing bucket data remains while backup data is restored without the "cannot create bucket" error.

Written by the indexing model from the issue text.