Update nginx-push-stream to follow nginx API changes.

Will support zlib 1.3+

Extend PATH to make xzcat available.

Extend PATH to make xzcat available.

Enforce python3 usage, as the newer version requires python3.

Empty values are considered not provided, so provide a template which
produces a file with a lone comment.

Instead of using openssl-1.0 and overriding the new versions to use
openssl, use openssl by default and override in the old versions to use
openssl-1.0

This reverts commit 852389ac.

This reverts commit bca3faca.

to avoid possible "no version information available" warnings.

See merge request nexedi/slapos!1453