Add provider attribute into gerrit events
Add trigger parameters using provider attribute in extended event

The below are added to triggered build.

GERRIT_NAME
GERRIT_HOST
GERRIT_PORT
GERRIT_PROTO
GERRIT_VERSION
Add 'url' key in provider attribute

Gerrit FrontEnd URL should be have by each gerrit hosts.
So add it to provider attribute.
In addition, improve class which treats the frontend URL.
Fix checkstyle issue and test code.
Fix null pointer access

Provider attribute as user defined has possibility not to exist.
So you need to check pointer before calling methods.
Add getGerritFrontEndUrlFor(event) then refactor related codes.
Add interface then implement to create map object
Merge pull request #52 from rinrinne/provider-attribute

Add trigger parameters using provider attribute