add metrics to existing ones

[mat1e]

Link to #150

This PR add somes metrics to existing ones and add improvments to existing functionnality.

Commons Points :

• instance : Jenkins instance url
• project_namespace : root path to the project (parent folder)

Jenkins Points :

• build_user : user who launch the build
• build_cause : Trigger type

Fix and improvments :

• Research of node_name in logs if the env var isn't setted
• Fix NoClassDefFoundException in tests by adding sshd plugin (it was a module before, see JEP-230)

[asimell]

Please also add relevant documentation to the available metrics documentation.

[asimell]

LGTM, but there's some merge conflicts. Could you address those before I merge this?