autopkgtests fail on arm64 (mongod failed to start)

[Christian Ehrhardt ]

Hi,
while pushing an SRU I realized that on arm the mongodb dep8 tests are somewhat shaky.
They sometimes work, but across xenial/yakkety/zesty it seems in about 50-70% of the cases I checked they all fail with the same issue.

Examples are [1], [2], [3] look at [4] and the other releases counterparts to get an overview.

I'm not sure how to handle this since it sometimes seems to work, I'm usually not a fan of disabling a test but as-is it is more a burden than a good quality indicator.

[1]: https://objectstorage.prodstack4-5.canonical.com/v1/AUTH_77e2ada1e7a84929a74ba3b87153c0ac/autopkgtest-zesty/zesty/armhf/m/mongodb/20161022_041956_ae678@/log.gz
[2]: https://objectstorage.prodstack4-5.canonical.com/v1/AUTH_77e2ada1e7a84929a74ba3b87153c0ac/autopkgtest-yakkety/yakkety/armhf/m/mongodb/20170519_163721_5664f@/log.gz
[3]: https://objectstorage.prodstack4-5.canonical.com/v1/AUTH_77e2ada1e7a84929a74ba3b87153c0ac/autopkgtest-xenial/xenial/armhf/m/mongodb/20161128_130514_1a771@/log.gz
[4]: http://autopkgtest.ubuntu.com/packages/mongodb/yakkety/armhf