Replace float with Decimal

Having float type as a base type for floating point parameters in
ipalib introduces several issues, e.g. problem with representation
or value comparison. Python language provides a Decimal type which
help overcome these issues.

This patch replaces a float type and Float parameter with a
decimal.Decimal type in Decimal parameter. A precision attribute
was added to Decimal parameter that can be used to limit a number
of decimal places in parameter representation. This approach fixes
a problem with API.txt validation where comparison of float values
may fail on different architectures due to float representation error.

In order to safely transfer the parameter value over RPC it is
being converted to string which is then converted back to
decimal.Decimal number on a server side.

https://fedorahosted.org/freeipa/ticket/2260

ipalib/encoder.py

@@ -20,6 +20,8 @@
 Encoding capabilities.
 """
 
+from decimal import Decimal
+
 class EncoderSettings(object):
     """
     Container for encoder settings.
@@ -77,7 +79,7 @@ class Encoder(object):
             return self.encoder_settings.encode_postprocessor(
                 var.encode(self.encoder_settings.encode_to)
             )
-        elif isinstance(var, (bool, float, int, long)):
+        elif isinstance(var, (bool, float, Decimal, int, long)):
             return self.encoder_settings.encode_postprocessor(
                 str(var).encode(self.encoder_settings.encode_to)
             )
@@ -131,7 +133,7 @@ class Encoder(object):
             return self.encoder_settings.decode_postprocessor(
                 var.decode(self.encoder_settings.decode_from)
             )
-        elif isinstance(var, (bool, float, int, long)):
+        elif isinstance(var, (bool, float, Decimal, int, long)):
             return var
         elif isinstance(var, list):
             return [self.decode(m) for m in var]