plugin installation fails if parent directory contains whitespace

Easy to reproduce, just install logstash in a directory where at least one parent dir component includes whitespace, then try to install a plugin.

The error appears to be in unquoted maven arguments causing a failure to generate jar dependencies, but I didn't debug it further once I realized the cause was the original scripting sin of not quoting paths. This is probably an upstream gem or ruby-maven bug, but logging it here as it makes logstash-plugin unusable.

Reproduce the failure by installing Logstash under a path containing whitespace, then run the logstash-plugin installation command. Inspect how the command constructs Maven arguments and determine whether the failure is in Logstash or the referenced ruby-maven/upstream dependency; done means plugin installation succeeds from such a path.

Written by the indexing model from the issue text.