* tests: add certbot-dns-rfc2136 integration tests
* dont use 'with' form of socket.socket
fixes py2 crash
* address some feedback:
- conftest: make DNS server a global resource
- conftest: add dns_xdist parameter into node config
- conftest: add --dns-server=bind flag
- conftest: if configured, point the ACME server to the DNS server
- dnsserver: make it sort-of compatible with xdist (future-proofing)
- context: parameterize dns-rfc2136 credentials file (future proofing)
- context: reduce dns-rfc2136 propagation time to speed up tests
- tox: add a integration-dns-rfc2136 target
- rfc2136: add a test/zone for subdelegation
- rfc2136: skip tests if no DNS server is configured
* try add integration-dns-rfc2136 to CI
* mock recursive dns via RPZ
* update --dns-server args and tox.ini args
* address more feedback:
- dns_server: rename rfc2136 creds file to .tpl
- dns_server: dont vary dns server port, instead we will vary zone names (#8455)
- dns_server: log error if bind9 fails to stop cleanly
- dns_server: replace assert with raise
- context: remove redundant _worker_id
- context: remove redundant cleanup override
- context: fix seek/flush in credentials context manager
- context: rename skip_if_no_server -> ...bind_server
- context: add newline EOF
* conftest: document _setup_primary_node sideeffects
* ci: rfc2136-integration from standard->nightly
* fix _stop_bind (function was renamed to stop)
* ignore errors from shutil.rmtree during cleanup
* dns_server: check for crash while polling
* remove --dry-run from rfc2136 test
