[2.1.x] Wrap log4j JsonTemplateLayout in pax-logging-log4j2

[jbonofre]

We just have to include a new log4j artifact in the wrapping (no additional dependency should be required).

[jbonofre]

Other additional log4j-* artifacts could be included this way.

[grgrzybek]

Thanks @svogt for the PR! I've added an integration test for this layout. Here's properties-based configuration:

json.template.log4j2.appender.file.type = File
json.template.log4j2.appender.file.name = file
json.template.log4j2.appender.file.fileName = target/logs-log4j2/json-template.log
json.template.log4j2.appender.file.append = true
json.template.log4j2.appender.file.layout.type = JsonTemplateLayout
json.template.log4j2.appender.file.layout.eventTemplateUri = ${karaf.etc.uri}/logging-template.json

json.template.log4j2.rootLogger.level = info
json.template.log4j2.rootLogger.appenderRef.file.ref = file

[grgrzybek]

Backports:

• to 2.0.x: #472
• to 1.12.x: #473
• to 1.11.x: #474