Ability to choose a different Elasticsearch version from ELASTIC_STACK_VERSION

[andsel]

In input|fitler|output Elasticsearch plugins there is a need to test the current plugin against various Elasticsearch versions.
For example using Logstash at version 7.x should be tested against Elasticsearch 6.8.latest.
The problem is that ELASTIC_STACK_VERSION is used to select both ES and LS versions, while we would test the latest LS 7.x with latest plugin version against different ES.
Consider this comment where the latest ES input plugin couldn't be installed on LS 6.8 but we have to test against ES 6.8.

An idea for the solution could be to have another optional environment variable (say ELASTICSEARCH_VERSION) which is considered to select the Elasticsearch only if specified else it uses the value of ELASTIC_STACK_VERSION.

[andsel]

When this is fixed re-enable the versions in .travis.yml removed with https://github.com/logstash-plugins/logstash-filter-elasticsearch/pull/150/files