Spec support for faking etcd TTLs

[SpComb]

Testing the behaviour of things like Kontena::Etcd::Writer which use etcd TTLs needs support for faking TTL expirty. For example, refresh should raise an exception if any nodes have expired before the refresh cycle kicks in.

Implementing this in the tests against a real ETCD_ENDPOINT=... would require support from the etcd server... however, we could feasibly implemenet support for faking etcd TTL expiry in the Kontena::Etcd::Test::FakeServer.

[SpComb]

Implemented for the Kontena::Etcd::Test::FakeServer in 04b27db