Simplify memory availability check, review tests

- Fix required memory for etcd hosts (10 -> 20 GB), as per
documentation.
- Some changes to make the code more similar to the similar
DiskAvailability check.
- Do not raise exception for hosts that do not have a recommended memory
value (those are ignored anyway through `is_active`, so that was
essentially dead code).
- Test that the required memory is the max of the recommended memories
for all groups assigned to a host. E.g. if a host is master and node, we
should check that it has enough memory to be a master, because the
memory requirement for a master is higher than for a node.

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