Add notSerializeDefault default field option

Add notSerializeDefault default field option

Default field with notSerializedDefault flag set is a default field which
will be serialized only if it's value differs from the default one. This
flag can be set by using notSerializedDefaultField field type instead of
defaultField field type.

This field is introduced in order to fix a bug of inconsistency between
haskell and python config modules which leads to inconsistent config
after ganeti updgrade.

Signed-off-by: Oleg Ponomarev <oponomarev@google.com>
Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>

Cherry-picked from: c0a2c62b9ad96c3e35cae0ffdcdf63a09164f537

Signed-off-by: Oleg Ponomarev <oponomarev@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>