Fix missing gitlabUserUsername definition for MR and Note triggers

[ljackiewicz]

It seems that gitlabUserUsername variable is not set in case of build triggered by MergeRequest event and Note (comments) event.

Additionally, gitlabUserName and gitlabUserEmail variables for these triggers (MR and Note) was changed to their real values from GitLab webhooks. In case of MR Event user is an author of specific MR, and in case of Note (Comments) Event user is an author of specific comment in MR, not author of last commit.

The proposed changes have been checked on a local GitLab instance (launched from src/docker).