Fixed "Unsupported bootstrap protocol" when specifying --no-bootstrap and added --ipfwd-rules option

Calling <tt>knife cs server create</tt> with <tt>--no-bootstrap</tt> causes 'Unsupported bootstrap protocol" error

The reason is that @bootstrap_protocol is not defined when specifying<tt>--no-bootstrap</tt>

Fix consists in checking <tt>config[:bootstrap]</tt> value before adding port forwarding rule for OS dependent bootstrap protocol.

In this way, a VM created with <tt>--no-bootstrap</tt> option has no portForwardingRule unless explicitly defined with <tt>--port-rules</tt>. Updated README also.

To correctly manage VM created with <tt>--static-nat</tt> and <tt>--no-bootstrap</tt> options also, I have added also <tt>--ipfwd-rules</tt> option. (Ref. http://cloudstack.apache.org/docs/api/apidocs-4.0.0/root_admin/createIpForwardingRule.html)

<tt>--ipfwd-rules PORT_RULES</tt>             <tt>Comma separated list of ip forwarding rules, e.g. '1024:10000:TCP,1024:2048,22'</tt>