Parameterize compress/decompress commands

[stankevich]

An improved version of #517.

Adds backupCompressCommand and backupDecompressCommand options that allow using any compressor/decompressor for backups. These options support arbitrary command line args as well.

Falls back to using gzip if the options are not provided.

---
apiVersion: mysql.presslabs.org/v1alpha1
kind: MysqlCluster
metadata:
  name: example
spec:
  # [...]
  backupCompressCommand:
    - pigz # https://zlib.net/pigz/
    - --processes 8
    - --stdout
  backupDecompressCommand:
    - pigz
    - --decompress
  # [...]

---

• I've made sure the Changelog.md will remain up-to-date after this PR is merged.

[AMecea]

You should include COMPRESS_COMMAND env on the sidecar container because new replicas (scaling-up) are added using the same mechanism as backups.

Let's check if only one of those field is specified and warn the user if so (logging the message should be enough).

[stankevich]

You should include COMPRESS_COMMAND env on the sidecar container

I might be missing something, but it looks like the sidecar container is the server and it simply runs xtrabackup without any compression. Adding a new replica streams an uncompressed backup directly to xbstream. Compression is currently only used in pushBackupFromTo, and decompression in cloneFromBucket. I have a local branch that enables compression when streaming but haven't had a chance to test it in somewhat real conditions yet.

check if only one of those field is specified and warn the user if so

Done: 0d59685

[AMecea]

LGTM 👍

Just one thing, please squash commits to keep a cleaner history. 😄
Thank you for your contribution!

[stankevich]

please squash commits to keep a cleaner history

Of course! Done.