Properly handle the case of installing only pure python modules

[jialeiwang]

It is not necessary for all users to compile C++ code, which sometimes is a headache to do on highly custom built systems. Currently you can specify env variable MOE_NO_BUILD_CPP to False to prevent compiling C++ code, but python interfaces for calling C++ code are still copied to the installed package, which is not desirable since you cannot call C++ code after all. The simple fix is to create a ifelse statement that chooses modules to install based on whether you want to compile C++ code or not.