Image deploy entrypoint and cmd parsing

[cezarsa]

The way tsuru handles reading the entrypoint from a image if a Procfile is not present is wrong. There are two major problems:

1. Only ENTRYPOINT is being read, where the correct approach would be to read both ENTRYPOINT and CMD directives and use them accordingly in the started containers.
2. Once we read the entrypoint we serialize it in a single string instead of using the original []string, this can cause a myriad of escaping problems and doesn't serve any real purpose. It would be much better to preserve the original []string.