Implement devices string parser

Relates to https://github.com/KostyaSha/yet-another-docker-plugin/issues/32

TODO add String parser for `Device.class` "/dev/sda:/dev/xvdc:r"

Parser
https://github.com/docker/docker/blob/6b4a46f28266031ce1a1315f17fb69113a06efe1/runconfig/opts/parse.go#L731-L764

Tests
https://github.com/docker/docker/blob/6b4a46f28266031ce1a1315f17fb69113a06efe1/runconfig/opts/parse_test.go#L468
