Add support for wal-g encryption environment variables (#649)

Close #636

ENVIRONMENT.rst

@@ -104,6 +104,7 @@ In case of S3, `wal-e` is used for backups and `wal-g` for restore.
 - **WALG_DELTA_MAX_STEPS**, **WALG_DELTA_ORIGIN**, **WALG_DOWNLOAD_CONCURRENCY**, **WALG_UPLOAD_CONCURRENCY**, **WALG_UPLOAD_DISK_CONCURRENCY**, **WALG_DISK_RATE_LIMIT**, **WALG_NETWORK_RATE_LIMIT**, **WALG_COMPRESSION_METHOD**, **WALG_BACKUP_COMPRESSION_METHOD**, **WALG_BACKUP_FROM_REPLICA**, **WALG_SENTINEL_USER_DATA**, **WALG_PREVENT_WAL_OVERWRITE**: (optional) configuration options for wal-g.
 - **WALG_S3_CA_CERT_FILE**: (optional) TLS CA certificate for wal-g (see [wal-g configuration](https://github.com/wal-g/wal-g#configuration))
 - **WALG_SSH_PREFIX**: (optional) the ssh prefix to store WAL backups at in the format ssh://host.example.com/path/to/backups/ See `Wal-g <https://github.com/wal-g/wal-g#configuration>`__ documentation for details.
+- **'WALG_LIBSODIUM_KEY**, **WALG_LIBSODIUM_KEY_PATH**, **WALG_PGP_KEY**, **WALG_PGP_KEY_PATH**, **WALG_PGP_KEY_PASSPHRASE** (optional) wal-g encryption properties (see [wal-g encryption](https://github.com/wal-g/wal-g#encryption))
 
 Azure Specific WAL-G Configuration
 `````

postgres-appliance/scripts/configure_spilo.py

@@ -766,7 +766,9 @@ def write_wale_environment(placeholders, prefix, overwrite):
                   'WALG_UPLOAD_CONCURRENCY', 'WALG_UPLOAD_DISK_CONCURRENCY', 'WALG_DISK_RATE_LIMIT',
                   'WALG_NETWORK_RATE_LIMIT', 'WALG_COMPRESSION_METHOD', 'USE_WALG_BACKUP',
                   'USE_WALG_RESTORE', 'WALG_BACKUP_COMPRESSION_METHOD', 'WALG_BACKUP_FROM_REPLICA',
-                  'WALG_SENTINEL_USER_DATA', 'WALG_PREVENT_WAL_OVERWRITE', 'WALG_S3_CA_CERT_FILE']
+                  'WALG_SENTINEL_USER_DATA', 'WALG_PREVENT_WAL_OVERWRITE', 'WALG_S3_CA_CERT_FILE',
+                  'WALG_LIBSODIUM_KEY', 'WALG_LIBSODIUM_KEY_PATH', 'WALG_PGP_KEY', 'WALG_PGP_KEY_PATH',
+                  'WALG_PGP_KEY_PASSPHRASE']
 
     wale = defaultdict(lambda: '')
     for name in ['PGVERSION', 'PGPORT', 'WALE_ENV_DIR', 'SCOPE', 'WAL_BUCKET_SCOPE_PREFIX', 'WAL_BUCKET_SCOPE_SUFFIX',