* separate constants and non-constants in 'lib/constants.py' to make
  it simple to replace their re-export definitions with a single
  re-export statement at the begining of the module

* fix access 'CONFD_REQFIELD_*' related constants in
  'tools/confd-client' to convert them to 'String', rather then doing
  it in 'lib/constants.py' in order to allow the replacement of the
  re-export definitions with a single re-export statement

Signed-off-by: Jose A. Lopes <jabolopes@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>