Amazon Linux: Added installing MongoDB and configuration file. Added conditions to identify linux distribution to python test script nodeos_run_test.py when running pgrep

after installing mongo c++ drivers changed code to cd into home dirctory before removing /tmp/mongodb-cxx-driver directory

minor correction in linux build scripts to accomodate small disk sizes that return float when running df -h

added building mongodb_plugin to cmake options, fixed potential issue with libtool when installing on a clean darwin OS and the default version of libtool

From a script by @joneric.

Added support for installing MongoDB via homebrew and compiling mongo-c-driver-1.9.3 && mongo-cxx-driver to darwin OS

compute the disk capacity based on block sizes and counts so ha terabyte capacity drives do not break the script