[autotest] Refactor reimage type selection to support >2 types.

One now passes a string that is tied to the name of the type of
reimaging that one would like to do. More reimaging types can now
be easily added.

BUG=chromium-os:38250
TEST=run_suite with trybot'd control.dummy changed to use reimage_type

Change-Id: I276556ff6822800cc846b20b564ffc36ec07e61b
Reviewed-on: https://gerrit.chromium.org/gerrit/42748
Reviewed-by: Aviv Keshet <akeshet@chromium.org>
Reviewed-by: Simran Basi <sbasi@chromium.org>
Commit-Queue: Alex Miller <milleral@chromium.org>
Tested-by: Alex Miller <milleral@chromium.org>
4 files changed