Support boolean attributes in json & text renderings

[mdutoo]

For now boolean attributes raise a "badarg" error (*), first because xs:boolean is not allowed in priv/schemas/occi.xsd .

However, boolean attributes are allowed in json (3.5 p8) and text (4.3 p6 "bool") renderings - as well as in OCCIware Cloud Designer.

Workaround : rather use an enum. This is actually a good practice from a semantic point of view (example : having a VM.isStarted attribute and later discovering that there are additional lifecycle states besides started and stopped).

(*) error :

=INFO REPORT==== 21-Sep-2016::22:25:44 ===
[erocci_backend_dbus] models()
=INFO REPORT==== 21-Sep-2016::22:25:44 ===
Calling <<"/">>:<<"org.ow2.erocci.backend.core">>.<<"Models">>([])
[{occi_parser_xml,parse,1,[{file,"src/occi_parser_xml.erl"},{line,33}]},{occi_rendering,parse,3,[{file,"src/occi_rendering.erl"},{line,60}]},{erocci_backend_dbus,parse_models,3,[{file,"src/erocci_backend_dbus.erl"},{line,312}]},{erocci_backend,handle_call,3,[{file,"src/erocci_backend.erl"},{line,410}]},{gen_server,try_handle_call,4,[{file,"gen_server.erl"},{line,615}]},{gen_server,handle_msg,5,[{file,"gen_server.erl"},{line,647}]},{proc_lib,init_p_do_apply,3,[{file,"proc_lib.erl"},{line,247}]}]

=ERROR REPORT==== 21-Sep-2016::22:25:44 ===
[line 16] parse error: {xml, <<"Unexpected error: badarg">>}