Refine Vagrantfile
    
    Major change is to create a dedicated Python virtual environment for MBS
    instead of installing required packages (listed in requirements) into
    system site-packages directory.
    
    This would be convenient for developer to install other Python packages
    via package manager, for example to install rhpkg. I found an issue when
    I installed rhpkg in original Vagrant machine, that is whole kobo
    package is installed via pip, and `dnf install rhpkg' will result in
    python2-kobo is installed and then it fails to import module
    kobo.rpmlib. The fix is I have to install python2-kobo-rpmlib via dnf
    again. With this change, this issue could be avoided. Two environments
    are separated rather than mixed together.
    
    Another two minor improvements base on this change are, after logging
    into the machine, 1) the virtual environment is activated automatically,
    and 2) change working directory to /opt/module_build_service.
    
    Signed-off-by: Chenxiong Qi <cqi@redhat.com>