use root partition for ephemeral data

Signed-off-by: dmitriy kalinin <dmitriy@pivotallabs.com>
extracted set up of data dir into a separate method on platform

Signed-off-by: dmitriy kalinin <dmitriy@pivotallabs.com>
split up big test for ephemeral disk set up

Signed-off-by: dmitriy kalinin <dmitriy@pivotallabs.com>
return err if data dir cannot be created

Signed-off-by: dmitriy kalinin <dmitriy@pivotallabs.com>