This is more of an information post than a question, because I noticed several unanswered questions along these lines while searching for a solution myself.
The problem occurs when trying to use bin/logstash-plugin to install a plugin behind a corporate firewall where the SSL certificate cannot be verified. One encounters a ruby error saying ERROR: Something went wrong when installing <PLUGIN>, message: certificate verify failed.
Well, I found a solution after some digging which seems to have worked for me.
You will need to edit a file called ${LOGSTASH_HOME}/lib/pluginmanager/install.rb
Somewhere near the beginning of the file (I placed it just after the "require" statements), place the following line of code:
OpenSSL::SSL.const_set(:VERIFY_PEER, OpenSSL::SSL::VERIFY_NONE)
Now when you run bin/logstash-plugin install you will get a big warning message from Ruby telling how you should not be disabling certificate verification, but your plugin should install correctly.