Delete TODO for hostname refactor

Isn't a simple refactor. At the time of writing a WIP branch existed.
ack grep for host_uri, found tested live code

We investigated the TODO and then used ack grep to look for live code and found
some. Hence the TODO was deleted.

$ ack -R "\.host_uri\ " *
main/python/yadtshell/actions.py
180:            action for action in self.actions if components[action.uri].host_uri in handled_hosts]

main/python/yadtshell/components.py
62:        self.host_uri = yadtshell.uri.create(yadtshell.settings.HOST, self.host)  # TODO(rwill): seems like dead code

main/python/yadtshell/update.py
79:                         and artefact.host_uri in handled_hosts
126:            if needs_host.host_uri != host.uri:

unittest/python/actions_tests.py
17:        mock_service0.host_uri = 'host://cowsay'
19:        mock_service1.host_uri = 'host://foobar'
21:        mock_service2.host_uri = 'host://foobaz'
39:        mock_service1.host_uri = 'host://foobar'
41:        mock_service2.host_uri = 'host://foobaz'
removed dead code

This code was commented out for a while and all unit tests have been passing.
Hence delete the code.
we need MissingComponent for readonly feature
solved TODO, refactored key.uri
removed unused logging
refactored use of _key_

The TODO suggested use of _key_ on RHS. Because MissingComponent takes a uri as
string as argument in the constructor we use _key_ in order to ensure a correct
type. Then, as a next step we refactored the call to _key_ into a variable and
reused this throughout the function.
removing unused code
refactor exit codes as constants

Two exit codes were still just plain integers. Refactor them to be module level
constants and place them where all the other exit codes are.
Remove TODO

This TODO is being deleted since the whole point of the create method is that
the type of component is not known. Hence it can not be refactored into
Component subclasses.
Remove TODO

The test tests what it is supposed to. Checks that the dangerous command is
executed if the user accepts.
yes, the command is dangerous
adding loggingoutput to DEBUG as requested
renamed and moved the exitcodes for the yadt minion to actionmanager.py
Merge pull request #11 from yadt/remove_todos

Remove todos