Issue #2659: release.py: Allow "nightly" as track name.
Issue #2659: release.py: Allow "nightly" as track name.

mozilla-mobile/focus-android,jonalmeida/focus-android,ekager/focus-android,jonalmeida/focus-android,jonalmeida/focus-android,ekager/focus-android,mozilla-mobile/focus-android,jonalmeida/focus-android,mozilla-mobile/focus-android,ekager/focus-android,mozilla-mobile/focus-android,ekager/focus-android,jonalmeida/focus-android,mozilla-mobile/focus-android,ekager/focus-android,ekager/focus-android,jonalmeida/focus-android,mozilla-mobile/focus-android