RFR: 1213: Generic jdk-cpu fixVersion

[Erik Joelsson]

This patch does two things.

1. Makes "jdk-cpu" a valid JdkVersion in Skara terms. This will allow us to use this generic version as fixversion for our jdk-cpu repos.
2. Makes it possible to configure "altfixversions" for a repository in the notifier. If an altfixversion is configured for a branch, then the notifier will not attempt to update any bugs or add any new backports if one of the altfixversions is already found to be fixed.

The second change is needed to prevent changes being merged from jdk to jdk-cpu from creating new backport records for every change in the jdk repo.

I removed the test "testIssuePoolOpenVersion" as it was an exact copy of the test preceding it "testIssuePoolVersion".

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Commit messages:
 - Allow generic jdk-cpu fixVersion
 - Make jdk-cpu a valid JdkVersion

Changes: https://git.openjdk.java.net/skara/pull/1234/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1234&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-1213
  Stats: 204 lines in 8 files changed: 149 ins; 11 del; 44 mod
  Patch: https://git.openjdk.java.net/skara/pull/1234.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1234/head:pull/1234

PR: https://git.openjdk.java.net/skara/pull/1234