added unit tests on buildslave.scripts.create_slave._makeBuildbotTac()
changed indention to follow PEP8
buildslave.test.misc.OpenFileMixin: specify default errno

In methods that patches open() to simulate open, read and write errors
use default errno for the exception raised.
added unit tests on buildslave.scripts.create_slave._makeInfoFiles()
renamed buildslave.test.misc.OpenFileMixin class to FileIOMixin

The FileIOMixin class name is more descriptive, as the mixin now patches
more then just open() function