Convert indentation from tabs to spaces (4)

Up until now I didn't see the point of using spaces for indentation.
However, the previous commit (a18bec3) was quite eye opening.
Given that python is an indentation aware language, the amount of
mistakes that went unnoticed because tabs and spaces were used
at the same time (tabs for indentation and spaces for alignment)
were unacceptable.

E101,W191 have been re-enable in the tox flake8 checker and
the documentation has been modified accordingly.

The following files have been left as-is:
* bootstrapvz/common/assets/extlinux/extlinux.conf
* bootstrapvz/common/assets/init.d/expand-root
* bootstrapvz/common/assets/init.d/generate-ssh-hostkeys
* bootstrapvz/common/assets/init.d/squeeze/generate-ssh-hostkeys
* bootstrapvz/plugins/docker_daemon/assets/init.d/docker
* bootstrapvz/providers/ec2/assets/bin/growpart
* bootstrapvz/providers/ec2/assets/grub.d/40_custom
* bootstrapvz/providers/ec2/assets/init.d/ec2-get-credentials
* bootstrapvz/providers/ec2/assets/init.d/ec2-run-user-data
* docs/_static/taskoverview.coffee
* docs/_static/taskoverview.less
* tests/unit/subprocess.sh

CONTRIBUTING.rst

@@ -143,10 +143,8 @@ guidelines. There however a few exceptions:
 * Max line length is 110 chars, not 80.
 * Multiple assignments may be aligned with spaces so that the = match
   vertically.
-* Ignore ``E101``: Indent with tabs and align with spaces
 * Ignore ``E221 & E241``: Alignment of assignments
 * Ignore ``E501``: The max line length is not 80 characters
-* Ignore ``W191``: Indent with tabs not spaces
 
 The codebase can be checked for any violations quite easily, since those rules are already specified in the
 `tox <http://tox.readthedocs.org/>`__ configuration file.

tox.ini

@@ -2,7 +2,7 @@
 envlist = flake8, unit, integration, docs
 
 [flake8]
-ignore = E101,E221,E241,E501,W191
+ignore = E221,E241,E501
 max-line-length = 110
 
 [testenv:flake8]