Fix #1129 - Add schedule validation check for task manifests in snapctl

[geauxvirtual]

Fixes #1129

Summary of changes:

• Validates a task from a task manifest includes a schedule and the schedule is not empty.
• Moves the version check for a task manifest into a better function that includes other validations.

Testing done:

• Verified passing a task manifest without a schedule returns an error.
• Verified passing a task manifest with an empty schedule returns an error.

@intelsdi-x/snap-maintainers

[jcooklin]

Should validateTask(t) be called after t.mergeCliOptions below?

[geauxvirtual]

The idea is to validate that both a schedule and workflow were included in the task manifest file and throw an appropriate error before the code attempts to use certain aspects of the created task struct so that the user will receive a better error than runtime error: invalid memory address or nil pointer dereference.

[jcooklin]

LGTM - however this PR doesn't fix #1130 as it is worded. The issue exists if curl is used.

[geauxvirtual]

Good catch. I can add to this PR some validation on the server side, or pull out the commits around #1130 and open a new PR with those commits plus the additional server side changes.