Fixes to compose syntax for volumes_from, volumes, and optional cpu parameter

[philipn]

• The cpu parameter in ECS is now optional. I think it was required before. So we don't emit an error message when it's not present.
• Allow the compose v2 style 'service:name:ro' syntax in volumes_from clauses.
• Allow the compose style ':rw' in volumes clauses.
• Emit readOnly on ECS volumesFrom clauses, which were previously skipped.

[cjerdonek]

Love it! 👍

[coveralls]

Coverage remained the same at 100.0% when pulling ab869b2 on philipn:support_rwo_volumes_from_and_cpu_optional into ff5741c on micahhausler:master.

[micahhausler]

Could you add an explanation parameter to this assert so the error is a little more clear? ``assert(len(parts) <=3, "volume string '{}' has too many colons".format(vol))` or something like that?

[micahhausler]

This is great! Thanks for submitting. Just one comment on the compose ingest_volumes_from, and we're good to go.

[micahhausler]

@philipn Any chance you could fix the indentation for the failing CI?

[coveralls]

Coverage remained the same at 100.0% when pulling f2890b3 on philipn:support_rwo_volumes_from_and_cpu_optional into 8a3ed35 on micahhausler:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 330dcc4 on philipn:support_rwo_volumes_from_and_cpu_optional into 224a596 on micahhausler:master.

[sveneh]

any blockers for this to be merged?