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>
diff --git a/scripts/update_payload/payload.py b/scripts/update_payload/payload.py
index 9450d02..8e50c1f 100644
--- a/scripts/update_payload/payload.py
+++ b/scripts/update_payload/payload.py
@@ -13,8 +13,8 @@
 from update_payload import block_tracer
 from update_payload import checker
 from update_payload import common
-from update_payload.error import PayloadError
 from update_payload import update_metadata_pb2
+from update_payload.error import PayloadError
 
 
 #
@@ -189,8 +189,6 @@
     if self.is_init:
       raise PayloadError('payload object already initialized')
 
-    # Initialize hash context.
-    # pylint: disable=E1101
     self.manifest_hasher = hashlib.sha256()
 
     # Read the file header.
@@ -232,11 +230,9 @@
         _DisplayIndentedValue('Build version', image_info.build_version)
 
     if self.manifest.HasField('old_image_info'):
-      # pylint: disable=E1101
       _DescribeImageInfo('Old Image', self.manifest.old_image_info)
 
     if self.manifest.HasField('new_image_info'):
-      # pylint: disable=E1101
       _DescribeImageInfo('New Image', self.manifest.new_image_info)
 
   def _AssertInit(self):