Make archive_command configurable using ENV variable.

[seppelucas]

The archive command is currently hardcoded in configure_spilo.py We would like to be able to overwrite the default settings with an extra environment variable.

Due to an bug in wal-g our wal-g wal-push commands in the postgres archive_command hang indefinitely. PostgreSQL itself will save and retry WAL files when the archive_command is unsuccessful (source). We would like to tweak the archive_command to include a timeout to automatically fail if the request to our backup location (In our case S3) takes too long.

I could find the archive_command in the postgres-operator docs here. In my opinion an easy fix would be to have an extra ENV variable that defaults to the current setup to be completely backwards compatible. But that would allow us and other users to tweak the command when necessary.