Allow bin/plugin to set -D JVM parameters

Currently the bin/plugin command did not allow one to set jvm parameters
for startup. Usually this parameters are not needed (no need to configure
heap sizes for such a short running process), but one could not set the
configuration path. And that one is important for plugins in order find
out, where the plugin directory is.

This is especially problematic when elasticsearch is installed as
debian/rpm package, because the configuration file is not placed in the
same directory structure the plugin shell script is put.

This pull request allows to call bin/plugin like this

bin/plugin -Des.default.config=/etc/elasticsearch/elasticsearch.yml -install mobz/elasticsearch-head

As a last small improvement, the PluginManager now outputs the directort
the plugin was installed to in order to avoid confusion.

Closes #3304