update_payload: Fix most of lint styling issues.

This patch fixes a lot of pylint issues in the update_engine scripts. Majority
of this changes are based on recommendation found in:
	https://www.chromium.org/chromium-os/python-style-guidelines

It is a good idea to do these changes now, because if there are many pylint
errors when performing 'repo upload', serious problems can be overshadowed by a
lot of noise and eventually cause problems.

These fixes include:
	- Fixing executable shebangs to /usr/bin/python2.
	- Fixing import-error problems by disabiling them.
	- Removing pylint disables that are not valid anymore.
	- Changing all imports to proper absolute import format.
	- Change the import of PayloadError from update_payload.PayloadError for
	  simplicity.
	- Add pydoc strings for functions and classes that were missing.

The remaining unchanged pylint problmes include:

	- The header files of these scripts are in CrOS copyright format, but the
      the cros lint hook is configured to AoSP copyright format.
	- The test* functions in unittests are not compatible with CamelCase format.

BUG=chromium:796338
TEST=unittests pass
TEST=start_devserver
TEST=cros flash
TEST=scripts/paycheck.py

Change-Id: I7eed4d1625eb7c510c7949fada120de5a6a26c7b
Reviewed-on: https://chromium-review.googlesource.com/834875


Commit-Ready: Amin Hassani <ahassani@chromium.org>
Tested-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Ben Chan <benchan@chromium.org>
Reviewed-by: Sen Jiang <senj@chromium.org>