BIGTOP-4528. Restore conf-select removed by BIGTOP-4063

[eubnara]

Description of PR

Restore conf-select that was unintentionally removed in BIGTOP-4063
(#1236, "Adapt bigtop-select to Python 3"). The PR was meant to port the
script to Python 3 but deleted it instead.

The script is still referenced by Ambari's BIGTOP stack:

• stacks/BIGTOP/*/properties/stack_tools.json points conf_selector at
  /usr/lib/bigtop-select/conf-select.
• ambari-common's resource_management.libraries.functions.conf_select
  invokes that path during stack-upgrade and config patching, and 30+
  service scripts (HDFS, YARN, HBASE, HIVE, KAFKA, SPARK, FLINK, LIVY,
  TEZ, ZEPPELIN, ZOOKEEPER, ALLUXIO, RANGER_KMS, ...) depend on it.

Changes:

• Restore conf-select with #!/usr/bin/env python3 (content otherwise
  identical to the pre-BIGTOP-4063 version; already Python 3 compatible).
• Re-add CONF_SELECTOR=conf-select and its install line in
  install_select.sh.
• Re-add Source3: conf-select and its %prep install line in
  bigtop-select.spec.

How was this patch tested?

• python3 -m py_compile bigtop-packages/src/common/bigtop-select/conf-select

For code changes:

• Does the title or this PR starts with the corresponding JIRA issue id (e.g. 'BIGTOP-3638. Your PR title ...')?
• Make sure that newly added files do not have any licensing issues. When in doubt refer to https://www.apache.org/licenses/

[eubnara]

@timyuer @sekikn @JiaLiangC
Could you review it please?