Provide easy way to access plugin configuration

Some plugins need some configuration. The idea is to store this
configuration in gerrit.config inside a "plugin" section having a
subsection per plugin. E.g.:

  [plugin "reviewers-by-blame"]
    maxReviewers = 3

A plugin can now easily access this configuration by:

  int maxReviewers = pluginConfigProvider
                        .get("reviewers-by-blame")
                        .getInt("maxReviewers", 0);

Change-Id: I4ef7386b7fa3cf4a986722acb8e16dee744697d1
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>
3 files changed