Rename docker-machine file to include 'plugin'

- We have some automation around enabling plugins which expects the word 'plugin'

plugins/available/docker-machine.plugin.bash

@@ -0,0 +1,11 @@
+cite about-plugin
+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="~/.docker/machine/machines/dev"
+  export DOCKER_TLS_VERIFY=1
+  export DOCKER_MACHINE_NAME="dev"
+fi