Remove import specific validation from tasks resource

Task-type specific validation should not be placed in the the
task resource as this will vary among providers. Validation of
the input values should occur in the task script and be reported
as part of the task status.

Change-Id: Ifbc1af868359e6bd51b18b10cd5402340438917a
Closes-bug: #1288337
Merge "Remove import specific validation from tasks resource"