Make testing more robust

[hawesie]

From @furushchev in #205

We currently use mongodb in 2 ways: using mongodb_server.py which spawns mongod instance or using system mongod (I called daemon in the previous comment) and in client side, there are two logics to check if mongodb server is alive or not for both cases. So we need to check both cases.
Also this package is released for indigo and kinetic which is run on ubuntu 14.04 / 16.04 respectively so it is better to test on both OS versions.
So my suggestion is to test on env:

| Ubuntu | ROS | mongodb server

-- | -- | -- | --
1 | 14.04 | indigo | mongodb_server.py
2 | 14.04 | indigo | system mongod
3 | 16.04 | kinetic | mongodb_server.py
4 | 16.04 | kinetic | system mongod