Aggregated `offer`,`answer`,`denied` message types into `roap-protocol`

Helps keeping the roap signaling consistent with other protocols and
makes clear the relationship between the different roap message types

* `offer`,`answer` and `denied` types are now called `roap-protocol`
* updated tests and mocks
* updated bootstrap servers
Added null/undefined check for empty signaling messages
renamed roap-protocol to signaling-protocol
Added type check to signaling messages

* checking for protocol type in python bootstrap server
* discarding message if protocol type is wrong
* checking if payload exists in signaling message
Merge branch 'master' into protocol-aggregation