Add GelfWriter

refs #7619
Add 'gelf' feature config

refs #7619
Add a type validafor for GelfWriter

refs #7619
Rebase against master changes, fix indent

refs #7619
GelfWriter: Fix default example port

refs #7619
GelfWriter: Add 'source' configuration attribute

refs #7619
GelfWriter: Add documentation

refs #7619
Merge branch 'feature/gelf-writer-7619'

fixes #7619