Change user and group validators to match shadow-utils

This sets the regex to [a-zA-Z0-9_.][a-zA-Z0-9_.-]{0,30}[a-zA-Z0-9_.$-]?

Also change the validators to return True/False

450613, 457124

ipa-python/radius_util.py

@@ -347,7 +347,7 @@ def validate_nastype(nastype, variable_name=None):
     return True
 
 def validate_desc(desc, variable_name=None):
-    if ipavalidate.Plain(desc) != 0:
+    if not ipavalidate.Plain(desc):
         print valid_desc_msg
         return False
     return True