RFR: 343: Use `ssh -G` for parsing ssh configuration files

[Erik Helin]

Hi all,

please review this patch that uses `ssh -G` instead of relying on my (somewhat
buggy) `ssh_config` parser. Resolving hosts defined in a `ssh_config` file is
actually fairly complex and my parser only handled a subset of scenarios. One
drawback is that we now rely on `ssh` being present on `PATH`, but that must be
the case if a user has a SSH path for a remote.

Testing:
- Manual testing using `git-webrev` with various paths

Thanks,
Erik

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Commit messages:
 - skara-343

Changes: https://git.openjdk.java.net/skara/pull/553/files
 Webrev: https://webrevs.openjdk.java.net/skara/553/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-343
  Stats: 351 lines in 7 files changed: 30 ins; 300 del; 21 mod
  Patch: https://git.openjdk.java.net/skara/pull/553.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/553/head:pull/553

PR: https://git.openjdk.java.net/skara/pull/553