Rename ChromeSDK SDKFetcher.  Defaults to not fetching base image.

1. Move to a model where the user of ChromeSDK specifies every component
they want to fetch.  The class becomes a generic fetcher for SDK
components, not limited to Chrome, thus rename it to SDKFetcher.

2. %CHROME_SDK_* becomes %SDK_*.

3. Default to not fetching the base image, since it expands to 3G of
disk space, and no scripts currently use it.  When 'cros chrome-flash'
requires the base image, it can use SDKFetcher to fetch it on demand.

BUG=None
TEST=unit tests.  ran locally.

Change-Id: I0850e9944ebb923a8511642d2a4580f3fcab2c01
Reviewed-on: https://gerrit.chromium.org/gerrit/43353
Reviewed-by: David James <davidjames@chromium.org>
Commit-Queue: Ryan Cui <rcui@chromium.org>
Tested-by: Ryan Cui <rcui@chromium.org>
diff --git a/scripts/deploy_chrome_unittest.py b/scripts/deploy_chrome_unittest.py
index 3f877c9..eb63dc2 100755
--- a/scripts/deploy_chrome_unittest.py
+++ b/scripts/deploy_chrome_unittest.py
@@ -214,7 +214,7 @@
     self.common_flags = ['--build-dir', self.build_dir,
                          '--board=lumpy', '--staging-only', '--cache-dir',
                          self.tempdir]
-    self.sdk_mock = self.StartPatcher(cros_chrome_sdk_unittest.ChromeSDKMock())
+    self.sdk_mock = self.StartPatcher(cros_chrome_sdk_unittest.SDKFetcherMock())
     self.PatchObject(
         osutils, 'SourceEnvironment', autospec=True,
         return_value={'STRIP': 'x86_64-cros-linux-gnu-strip'})