fix: quoting s3 secret key

[prometherion]

Closes #43.

Please @Heiko-san, may I ask you for a review of this?

I used the following values:

backup:
  enabled: true
  s3:
    url: http://mys3storage:9000
    bucket: mybucket
    retention: "--expiry-days 7" #"
    accessKey: minio
    secretKey: 'xY%foo$ba)R'

And the resulting Job specification is the following:

              - |-                   
                cd /opt/etcd-dump                                                                        
                if $MC alias set myminio http://mys3storage:9000 minio "xY%foo$ba)R" \
                && $MC ping myminio -c 3 -e 3 ; then 
                  echo -e "\nUploading snapshot(s):" 
                  $MC cp mysnapshot_*.db myminio/mybucket
                else        
                  echo -e "\nERROR: S3 storage could not be configured;\nCheck your S3 URL/Credentials or network connectivity"
                  exit 1
                fi

[ptx96]

LGTM!

[prometherion]

Switched over environment variables, please, @ptx96 @Heiko-san, can you review it?

[prometherion]

@ptx96 latest commits are supporting the credentials retrieval from an external object, such as a Secret.

[Heiko-san]

Both approaches (secret ref & quoted value) work great in our setup!
Thanks :)