Wait on consumer_info promise in login (bug 1038985, bug 1039868)

We need to wait for consumer-info because it's the source of the
waffle switches - if we don't, then sometimes the fallbackFxA check
is done too early and returns false when it's enabled.
Merge pull request #540 from diox/fallback-fxa-race-condition

Wait on consumer_info promise in login (bug 1038985, bug 1039868)