Fix gem installer tests to enable unpinning the version of bundler
This commit removes changes the gem installer to use real gems, rather than
use `allow_instance_of` during testing, which appears to be problematic with the
latest version of bundler
A pack in this context is a *bundle* of plugins that can be distributed outside of rubygems; it is similar to what ES and kibana are doing, and
the user interface is modeled after them. See https://www.elastic.co/downloads/x-pack
**Do not mix it with the `bin/logstash-plugin pack/unpack` command.**
- it contains one or more plugins that need to be installed
- it is self-contains with the gems and the needed jars
- it is distributed as a zip file
- the file structure needs to follow some rules.
- As a reserved name name on elastic.co download http server
- `bin/plugin install logstash-mypack` will check on the download server if a pack for the current specific logstash version exist and it will be downloaded, if it doesn't exist we fallback on rubygems.
- The file on the server will follow this convention `logstash-mypack-{LOGSTASH_VERSION}.zip`
- As a fully qualified url
- `bin/plugin install http://test.abc/logstash-mypack.zip`, if it exists it will be downloaded and installed if it does not we raise an error.
- As a local file
- `bin/plugin install file:///tmp/logstash-mypack.zip`, if it exists it will be installed
Fixes#6168
