coverage: allow overriding coverage threshold (#13236)
coverage: allow overriding coverage threshold (#13236)

Commit Message: allow overriding coverage threshold
Additional Description: allows third party runners (like Envoy Mobile) to set their own coverage thresholds independent of Envoy's threshold.
Risk Level: low - nothing changing in the Envoy CI run.
Testing: CI, and tested overriding in Envoy Mobile.

Signed-off-by: Jose Nino <ca94e32c82b10963d0ce6eadf427a615ff7d1c0b@lyft.com>
envoyproxy/envoy-wasm,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,lyft/envoy,envoyproxy/envoy,lyft/envoy,lyft/envoy,lyft/envoy,lyft/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy-wasm,envoyproxy/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy,lyft/envoy,envoyproxy/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy,envoyproxy/envoy-wasm