bin/logstash-plugin update should not include major versions changes by default

[robbavey]

Currently, running bin/logstash-plugin update will update plugins to the latest version possible, regardless of whether a major version has changed in the plugin.

This may cause issues, and cause the plugin manager to install a version of the plugin which is incompatible with the associated version of Logstash.

The default behavior for plugin updates should be to attempt to install the latest version in the same minor rather than the absolute latest version, but allow that update to be made if required.

[jsvd]

diff --git a/lib/bootstrap/bundler.rb b/lib/bootstrap/bundler.rb
index 325222021..eb5d6b08e 100644
--- a/lib/bootstrap/bundler.rb
+++ b/lib/bootstrap/bundler.rb
@@ -266,6 +266,7 @@ module LogStash
         arguments << expand_logstash_mixin_dependencies(options[:update])
         arguments << "--local" if options[:local]
         arguments << "--conservative" if options[:conservative]
+        arguments << "--minor"
       elsif options[:clean]
         arguments << "clean"
       elsif options[:package]

This is the changeset needed to affect the update command.
But it's necessary to:

a) Introduce a new parameter to the update command such as :constraint, that defaults to "minor", but can also be "patch" or "major".
b) Understand how it behaves with the existing conservative parameter