provision/juju: added handler instance to the package

This handler will be used to consume messages from the queue.

Related to #247.
provision/juju: fix bound bug in handle

It should check the bound before asking for something in the slice. Dumb
error, sorry :-(
provision/juju: use the queue to register new units in the load balancer

Whenever a new unit is added to the app, JujuProvisioner will put a new
message in the queue, containing the name of the app and the name of the
new units. When handling this message, the handler will use juju to get
the Amazon instance-id of the units and register them in the load
balancer attached to the app.

Related to #247 (almost there, now we need to add new units in the load
balancer when we create a new app).
provision/juju: handle queue messages for all units in an app

When the add-unit-to-lb messages receive only one parameter (the name of
the app), it gets all units of the app and add to the load balancer.

Related to #247.
provision/juju: make test more reliable

String sorting may catch you :-)

i-10 < i-9.
provision/juju: also start handler in dry-run mode in normal suite
provision/juju: handle units that still don't have an ID

Whenever a unit doesn't have an instance id yet, the handler will add it
to a list of units that don't have ID. If none of the units have an ID,
the message is released, otherwise, all units that have an ID are
registered in the load balancer, the original message is deleted and a
new message is generated in the queue, for later processing.

Related to #247.
provision/juju: register units in ELB when provisioning new apps

A special message containing only the name of the app indicates that all
units of that app should be registered in the load balancer.

Closes #247.
provision/juju: clean queue before running tests

Just protecting against external processes...
provision/juju: fix AddUnit

Looks like I'm just stupid.

Actually closes #247.