Check existence of $HELM_HOME/plugins before installing plugin
Check existence of $HELM_HOME/plugins before installing plugin

Fixes https://github.com/kubernetes/helm/issues/2488

sushilkm/helm,helm/helm,thomastaylor312/helm,adamreese/deployment-manager,fibonacci1729/helm,jascott1/helm,technosophos/k8s-helm,kdada/helm,rbwsam/helm,appscode/helm,fibonacci1729/helm,michelleN/deployment-manager,adamreese/deployment-manager,helm/helm,michelleN/deployment-manager,SlickNik/helm,jascott1/helm,rbwsam/helm,eicnix/helm,technosophos/deployment-manager,michelleN/deployment-manager,adamreese/helm,eicnix/helm,FengyunPan/helm,SlickNik/helm,flynnduism/helm,technosophos/deployment-manager,adamreese/deployment-manager,technosophos/deployment-manager,FengyunPan/helm,kdada/helm,technosophos/k8s-helm,adamreese/deployment-manager,adamreese/deployment-manager,technosophos/deployment-manager,adamreese/helm,bacongobbler/helm,bacongobbler/helm,appscode/helm,technosophos/deployment-manager,flynnduism/helm,sushilkm/helm,thomastaylor312/helm,michelleN/deployment-manager