Add support for on-demand coverage on Shippable.

Triggered by setting the COVERAGE environment var to a non-empty value.
Alternatively the string `ci_coverage` can be added to a commit message.

test/utils/shippable/shippable.sh

@@ -23,6 +23,12 @@ pip list --disable-pip-version-check
 
 export PATH="test/runner:${PATH}"
 export PYTHONIOENCODING='utf-8'
+export COVERAGE="${COVERAGE:-}"
+
+# run integration coverage if 'ci_coverage' is in the commit message or the COVERAGE var is non-empty
+if [[ "${COMMIT_MESSAGE}" =~ ci_coverage ]] || [ -n "${COVERAGE}" ]; then
+    export COVERAGE="--coverage"
+fi
 
 # remove empty core/extras module directories from PRs created prior to the repo-merge
 find lib/ansible/modules -type d -empty -print -delete