Change the internal default values of local image paths

This commit refactors the code without changing the functionality.
If --local-image is not specified, the argument defaults to None.
If --local-image is not followed by a path, the argument defaults to
FIND_IN_BUILD_ENV. This commit also fixes most pylint errors in the unit

Test: acloud-dev create --local-instance --local-image
Test: acloud-dev create --local-instance --build-id 7050048 \
      --build-target aosp_cf_x86_64_phone-userdebug
Bug: 176270532
Change-Id: I97253d07af9c6a9be91e32c7a0e64f995d833c12
9 files changed