RFR: Add support for using ssh cloning with GitLab

[Robin Westberg]

Add support for configuring a GitLab forge to use ssh for cloning repositories using a private ssh key.

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Commit messages:
 - Add support for using ssh cloning with GitLab

Changes: https://git.openjdk.java.net/skara/pull/1057/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1057&range=00
  Stats: 78 lines in 4 files changed: 56 ins; 4 del; 18 mod
  Patch: https://git.openjdk.java.net/skara/pull/1057.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1057/head:pull/1057

PR: https://git.openjdk.java.net/skara/pull/1057