Cage service should expect config file instead of binary

[DonatJR]

• service or manager, depending on whether I tackle this or #21 first
• kind of depends on #11 because we need a final version of how the config (v1) should look like. for now I'm going to assume that at least a binary_path can be found in it
• part of #37 and continuation of #10

[DonatJR]

done with #37