Resolve "Use pylint for linting"

Closes #239 (closed)

Linted through the code base and made small changes to various parts of the project.

NOTE: There are still some pylint warning messages disabled, this is either because:

1. No obvious changes could be made to "improve" the standard of the code, or
2. The changes that were made to address this message lead to build errors.

Using pylint

To lint the codebase run ./setup.py test --addopts '--pylint -m pylint'

• This will provide warnings when the linter comes across a potential error or style issue. They are generally relatively easy to fix/change.
• Please note that some warnings have been permanently disabled in the .pylintrc configuration file
• If the warning for a line of code is irrelevant disable it using: # pylint: disable=<warning-name>

buildstream/_ostree.py

@@ -66,7 +66,7 @@ def ensure(path, compress):
 #    commit (str): The commit checksum to checkout
 #    user (boot): Whether to checkout in user mode
 #
-def checkout(repo, path, commit_, user=False):
+def checkout(repo, path, commit, user=False):
 
     # Check out a full copy of an OSTree at a given ref to some directory.
     #