add remote plugin stash

the Stash stuct has a PrivateKey attribute that is currently set in the stash init.go file and expects there to be a "drone.pem" file in the server dir. Not pretty! But I'm not sure how you would like to handle config options for plugins. I did not want to add this field for all remotes without checking with you first.

This private key is needed for oauth, so not necessarily related to sslkey/sslcert options, as we will actually not be running our drone instance under ssl.

Also note that this stash implementation works against self hosted stash instances, so path not always stash.atlassian.com