Use $OSTYPE instead of uname to speed things up

The $OSTYPE variable is set at ZSH compile time and can be safely used
to determine the OS of the system. e.g. darwin (os x)
Support dynamically defined Vagrant machines
Merge pull request #2744 from docwhat/ostype-not-uname

Use $OSTYPE instead of uname to speed things up
Merge pull request #2811 from stucki/multiple-vagrant-machines

Support dynamically defined Vagrant machines