make unitfile independent of docker.service

[lsm5]

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This allows both docker and docker-latest to use novolume-plugin without any changes in novolume-plugin's unitfile.

Instead docker and docker-latest's unitfiles will have a dependency on docker-novolume-plugin like so:

After=docker-novolume-plugin.socket Wants=docker-novolume-plugin.socket

Signed-off-by: Lokesh Mandvekar [email protected]

[lsm5]

@runcom @rhatdan ptal