Cleaned up JUnit report generation on install (#6977)

* Cleaned up JUnit report generation on install

The generation of a JUnit report was previously part of the install
command. This commit factors the logic into its own module, and uses
a template for the generation of the report.

It also improves report generation, that now can deal with multiple
specs installed at once. Finally, extending the list of supported
formats is much easier than before, as it entails just writing a
new template.

* Polished report generation + added tests for failures and errors

The generation of a JUnit report has been polished, so that the
stacktrace is correctly displayed with Jenkins JUnit plugin. Standard
error is still not used.

Added unit tests to cover for installation failures and installation
errors.