[JENKINS-21494] Add server credentials in env-var settings

The plugin provides global and user settings in Maven Jobs:
- during the Maven build,
- referenced in environment variables, for other job actions such as a Sonar analysis (my use case)

A recent feature also adds credentials defined using the Jenkins Credential plugin, but just in the first case (for the Maven build).
This pull request adds credential mappings in the settings generated separately to be referenced in environment variables.

I tested in both user and global settings, but not knowing much of the Jenkins plugin tests, I did not have the time to investigate on providing unit tests. Maybe later.