provision/docker: set app status to ready on Provision

Related to #450.
collector: set the app to ready when one of its units is started

It only touches the value when it's empty. If the provisioner set some
strange value or says that it's ready, collector will respect it.

Related to #450.
app: refactong methods for command execution

We're getting there :-)
app: don't check app availability on runHook