Using common method 'bool_from_string' from oslo strutils

Using common method 'bool_from_string' from oslo strutils to replace
utils.string_to_bool.

partially implements blueprint common-client-library-2

Change-Id: I23924db3000feadcfe823c6cc979ea9752a13fa9

glanceclient/common/utils.py

@@ -48,7 +48,7 @@ def schema_args(schema_getter, omit=[]):
     typemap = {
         'string': str,
         'integer': int,
-        'boolean': string_to_bool,
+        'boolean': strutils.bool_from_string,
         'array': list
     }
 
@@ -176,10 +176,6 @@ def is_authentication_required(f):
     return getattr(f, 'require_authentication', True)
 
 
-def string_to_bool(arg):
-    return arg.strip().lower() in ('t', 'true', 'yes', '1')
-
-
 def env(*vars, **kwargs):
     """Search for the first defined of possibly many env vars