Validate etc/gerrit.config file upon PUT request

Make sure that the content of an etc/gerrit.config file that is uploaded
by this plugin is syntactically correct by asserting that the
config can be loaded. No semantic validation is performed, however.

Change-Id: I400cca45ddb47e44aa304a304aabc00109736ab3
Signed-off-by: Adrian Görler <adrian.goerler@sap.com>