Modify mongo cluster to use FQDN for nodes in integration

[rtrinque]

We need this to run the new platform based on ECS as it is unable
to resolve shortened host names.

[sengi]

Btw if you link to the Trello card in the PR description it'll update the checklist in the card for you.

[bilbof]

Diff from puppet run:

 function replicaSetMembers() {
-  var members = ["mongo-1","mongo-2","mongo-3"];
+  var members = ["mongo-1.integration.govuk-internal.digital","mongo-2.integration.govuk-internal.digital","mongo-3.integration.govuk-internal.digital"];
   var i = 0;