Add binary prefix for version 5.x

[veger]

As requested in #182 a PR for the 5.x branch.

Copied from #178:

The plugin executable for Elasticsearch version 5.0.0, is renamed elasticsearch-plugin.

This PR uses the new executable name when es_version indicates that version 5 (or higher) is used.

Some other minor plugin-related changes:

• Removed sed -n '1!p' | cut -d '-' -f2- as the executable just returns a list of installed plugins (or is this also new in version 5.x?)
• Added task name for 'installed plugins check'
• Result of 'remove plugins' task is stored in plugin_removed

Note that I have updated the xpack/* files as well, but this is untested. Since it also contains some shell filters (tail -n +2), I would assume it does not work with version 5 as well...

[elasticmachine]

Jenkins standing by to test this. If you aren't a maintainer, you can ignore this comment. Someone with commit access, please review this and clear it for Jenkins to run.

[jakommo]

Jenkins, test it!

[veger]

Fixed situation where es_plugins is undefined, resulting in es_binary_prefix being undefiend as well. Now es_binary_prefix defined no matter what.

[jakommo]

Jenkins, test it!

[veger]

While setting up the (failing) test, the rspec-retry ruby gem was not found, is it possible that this influences the CentOS 7 standard-1x test? (as this is now the only test that fails)

[jakommo]

Looks like the real problem was that the node did not start:

Node listening
listening in port 9200 (FAILED - 1)

version check
should be reported as version 1.7.3 (FAILED - 2)

Not sure if this is caused by the rspec-retry ruby gem.
let's run the test again .

[jakommo]

Jenkins, test it!

[veger]

Looks like it was an hiccup in the test scripts

[jakommo]

Merged, thanks again @veger