Prevent docker network's collision at parallel drone exec execution

[suzuki-shunsuke]

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By default executing multiple drone exec commands in parallel raises docker network's collision because prefix is static.

$ drone exec --local & drone exec --local
[1] 42934
2018/06/06 01:58:12 Error response from daemon: network drone_default is ambiguous (4 matches found on name)
2018/06/06 01:58:12 Error response from daemon: Conflict. The container name "/drone_step_0" is already in use by container "464a29b0726d6ff1a352d81df9c837330501085be550bb16abac3d338dfad887". You have to remove (or rename) that container to be able to reuse that name.
[1]  + exit 1     drone exec --local

On the other hand, drone server can execute parallel builds without docker network collisions because random prefix is used.

https://github.com/drone/drone/blob/29785b86f6534ded974120de0fcf7c21397a9d0d/server/hook.go#L552

So I add the random string to prefix to prevent the collisions.

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