RFR: Add support for using ssh cloning with GitLab [v2]

[Robin Westberg]

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

Robin Westberg has updated the pull request incrementally with one additional commit since the last revision:

  Use different configuration syntax

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Changes:
  - all: https://git.openjdk.java.net/skara/pull/1057/files
  - new: https://git.openjdk.java.net/skara/pull/1057/files/169c0cae..bf926255

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=skara&pr=1057&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=skara&pr=1057&range=00-01

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 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