Source docker-machine environment variables rather than hardcoding

Signed-off-by: Frank Kotsianas <fkotsian@pivotal.io>

plugins/available/docker-machine.plugin.bash

@@ -4,8 +4,5 @@ about-plugin 'Helpers to get Docker setup correctly for docker-machine'
 # Note, this might need to be different if you use a machine other than 'dev',
 # or its configured for a different IP
 if [[ `uname -s` == "Darwin" ]]; then
-  export DOCKER_HOST="tcp://192.168.99.100:2376"
-  export DOCKER_CERT_PATH="$HOME/.docker/machine/machines/dev"
-  export DOCKER_TLS_VERIFY=1
-  export DOCKER_MACHINE_NAME="dev"
+  eval "$(docker-machine env dev)"
 fi