Create a flyway index file and verify correctness (#819)

* Create a flyway index file and verify correctness

Create an index file (flyway.txt) containing the names of all of the flyway
files and verify that it is ordered and in sync with the actual contents of
the flyway directory.  Also provide a target (generateFlywayIndex) to
automatically generate it.

The purpose of flyway.txt is to cause a merge conflict in the event that two
different developers add a flyway file with the same sequence number, an event
which has occurred multiple times.

build.gradle

@@ -191,7 +191,7 @@ allprojects {
 }
 
 task runPresubmits(type: Exec) {
-  executable '/usr/bin/python'
+  executable '/usr/bin/python3'
   args('config/presubmits.py')
 }