Allow PasswordGenerator to use different alphabets

Per mcilwain's suggestion in the LRP design doc, LRP tokens should use a Base58 alphabet. I'll move PasswordGenerator out of the tools package and into a utils class in a future CL, as we'll want to use this generator in the LrpToken class itself rather than relegate the token definition to a tool.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=132358363

javatests/google/registry/tools/SetupOteCommandTest.java

@@ -41,7 +41,8 @@ public class SetupOteCommandTest extends CommandTestCase<SetupOteCommand> {
 
   ImmutableList<String> passwords = ImmutableList.of(
       "abcdefghijklmnop", "qrstuvwxyzabcdef", "ghijklmnopqrstuv", "wxyzabcdefghijkl");
-  FakePasswordGenerator passwordGenerator = new FakePasswordGenerator("abcdefghijklmnopqrstuvwxyz");
+  DeterministicStringGenerator passwordGenerator =
+      new DeterministicStringGenerator("abcdefghijklmnopqrstuvwxyz");
 
   @Before
   public void init() {