gclient setdep used to assume that the version of a CIPD package was always
of the form 'version:<something>'.
However, the following are all valid:
  * SHA1 hashes that specify a single instance
  * tags (tagname:tagvalue)
  * References (e.g. 'latest') that are dynamically resolved by the CIPD server.

This change adds support for all of them:
  * GetCIPD simply returns whatever the 'version' entry is, without assuming
    it begins with 'version:'.
  * SetCIPD sets 'version' ta whatever it is passed, without prepending 'version:'
    at the beginning.

Bug: 858978
Change-Id: I53669c5df7fb51cde42e0af271139b5719a47622
Reviewed-on: https://chromium-review.googlesource.com/1120943Reviewed-by: Aaron Gable <agable@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>