utils.astring: Add preferred ENCODING

There is no right encoding, but the `locale.getpreferredencoding` suits
most cases. Let's avoid the usually unnecessary `locale` import and
people struggling to remember the most appropriate method and store it
in `avocado.utils.astring`.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

avocado/utils/astring.py

@@ -26,6 +26,7 @@ And not notice until their code starts failing.
 """
 
 import itertools
+import locale
 import re
 import sys
 import string
@@ -35,6 +36,12 @@ from six.moves import zip
 from six.moves import xrange as range
 
 
+#: On import evaluated value representing the system encoding
+#: based on system locales using :func:`locale.getpreferredencoding`.
+#: Use this value wisely as some files are dumped in different
+#: encoding.
+ENCODING = locale.getpreferredencoding()
+
 #: String containing all fs-unfriendly chars (Windows-fat/Linux-ext3)
 FS_UNSAFE_CHARS = '<>:"/\\|?*;'