only export MongoDB dependency for shared mongodb-library

libmongoclient.a uses quite a number of other libs and the exact requirements
can't be read from a cmake/pc file. Therefore it makes more sense to keep the
dependency hidden from ROS when we use the static lib. libwarehouse_ros then
provides all required functions.
... This is a bit like creating a libmongoclient.so, but the whole problem 
exists because debian/ubuntu don't provide this one, right?

The shared library can - and has to - be exported as a dependency to ROS.

This pull-request should cleanup the problem @tfoote noticed in #14.

Please have a look @isucan.