issueManagement/system not used by changes:changes, must override

### Affected version

3.0.0-M3

### Bug description

When generating a site, the generating a page for `changes.xml` does NOT use the POM's `/issueManagement/system` value to select, for example "JIRA" and instead uses the "defauit", which generates broken URLs on that page.

For example:
```
git clone https://github.com/apache/commons-validator.git
cd commons-validator
git checkout 4c9a0660a0d29bc7e9891f24be86f9539f42e419
mvn clean verify site -DskipTests
open target/site/changes.html#a1.10.1
```
Youi'll see the link to [VALIDATOR-502](https://issues.apache.org/jira/browse/ViewIssue.jspa?key=VALIDATOR-502) with the href `https://issues.apache.org/jira/browse/ViewIssue.jspa?key=VALIDATOR-502` which uses the "default" key instead of the "JIRA" key defined in `/issueManagement/system`

Note that `/issueManagement/system` is inherited from the parent POM, which works since the JIRA report is OK; it's the changes.xml report that's broken.

I see in `changes.mdo` that an action supports a `system` attribute. Shouldn't that inherit `/issueManagement/system`? Otherwise, you need to specify system on each action, which seems redundant.

Start with the changes.xml report generation and the action definition in changes.mdo, then reproduce the issue in commons-validator with mvn clean verify site -DskipTests. Trace how the inherited POM /issueManagement/system value is handled for changes actions; done means the generated changes.html uses the configured JIRA key rather than the default URL.

Written by the indexing model from the issue text.