template availablility and readiness

[a-gerhard]

There is a difference between template availability and readiness:

• available: hosts that know the correct information about the template
• ready: hosts that have the template ready on their disk

the stats should be:

• ready on backend
• available on hosts (out of all hosts which currently work)
• ready on hosts (out of hosts with available=True)

host selection for elements should filter for availability, not readiness host selection should prefer hosts with template ready