inject VERSION_QUALIFIER into the build to be displayed to users

[jsvd]

previously in 5.0.0-8.0.0 the version qualifier wasn't present in the build artifacts.

To facilitate distinction between GA and pre-releases like betas and rcs, we'd like for the VERSION_QUALIFIER to be present in the build artifacts.

[jsvd]

Issues:

1. Artifact tasks copy the versions.yml to two places:

./logstash-core/versions-gem-copy.yml
./logstash-core-plugin-api/versions-gem-copy.yml

2. we have the same code for version loading 3 times:

./logstash-core/lib/logstash/version.rb:  ALL_VERSIONS = YAML.load_file(File.expand_path("../../versions-gem-copy.yml", 
./logstash-core/lib/logstash-core/version.rb:  ALL_VERSIONS = YAML.load_file(File.expand_path("../../versions-gem-copy.yml", File.dirname(__FILE__)))
./logstash-core-plugin-api/lib/logstash-core-plugin-api/version.rb:  ALL_VERSIONS = YAML.load_file(File.expand_path("../../versions-gem-copy.yml", File.dirname(__FILE__)))

3. During normal Logstash startup logstash-core-plugin-api is the one that is loaded first, so it loads versions from ./logstash-core-plugin-api/versions-gem-copy.yml.

4. Depending on the command/method to see the version, we see different versions

a) bin/logstash --version will load the value from cat ./logstash-core/versions-gem-copy.yml | yq .logstash
b) bin/logstash --debug --version will load the value from cat ./logstash-core-plugin-api/versions-gem-copy.yml | yq .logstash
c) curl localhost:9600 | jq .version will load the value from cat ./logstash-core-plugin-api/versions-gem-copy.yml | yq .logstash-core (constant LOGSTASH_CORE_VERSION).