Graylog fails (not fail-fast) to start if mongodb is not initially reachable

Hi guys,

**My scenario is as follows:**
1. MongoDB is DOWN
2. Graylog is starting
3. Graylog shows in logs that it fails to connect to MongoDB
4. Graylog behaves as if it would recover once the MongoDB is UP. It can be observed in logs
5. MongoDB is started
6. Graylog opens a connection to MongoDB but then fails with Guice error while trying to dependency-inject
7. Graylog says Terminating and kills itself

**My environment:**
OS: SLES 11.4 (sles)
JRE: Oracle Corporation 1.8.0_144 on Linux 3.0.101-108.35-default
Graylog server 2.4.3+2c41897
Arch: amd64
MongoDB: 3.0.0
Elasticsearch: 5.6.9
MongoDB + Elasticseacrh + Graylog are all on the same host
Every URL in Graylog configuration is pointing to 127.0.0.1

[Here](https://github.com/Graylog2/graylog2-server/files/2119681/issue.log) is the log of the above scenario

I would expect that Graylog should eventually connect successfully to MongDB or fail-fast in this case. What do you think? Is this possible at all?