Split "variant" terminology into "variant" and "variation"

"Variant" has been used to mean two slightly concepts in Blueprint.
The first and most obvious is a variant of a module, meaning a list
of all the ways that on version of a module differs from other
versions of the same modules, for example "arch:arm, link:static".
The other is for the specific way that a version of a module differs,
"arch:arm".

Rename all of uses of "variant" as the second meaning to "variation",
and update the documentation to clarify "variant" vs. "variation".
This modifies the build logic api to convert CreateVariants to
CreateVariations.

Change-Id: I789ef209ae6ddd39ec12e0c032f9f47f86698fe6