Fix server version matching logic on integration suite

[dkropachev]

1. Introduce BackendType.Scylla
2. Introduce Environment.GetEnvironmentVariable("SCYLLA_VERSION")
3. Fix all the logic around TestClusterManager.CurrentBackendType to work properly with BackendType.Scylla
4. Implement TestScyllaVersion
5. Fix TestCassandraVersion and TestBothServersVersion to march Scylla when cs version is less then 4.0.0
6. Fix TestDseVersion to not to match scylla

Update driver-matrix fixes when it is merged

[sylwiaszunejko]

2. Introduce Environment.GetEnvironmentVariable("DSE_VERSION")

I don't quite understand what this point is about

[dkropachev]

2. Introduce Environment.GetEnvironmentVariable("DSE_VERSION")

I don't quite understand what this point is about

I am sorry, what I meant is to introduce SCYLLA_VERSION and make use of it in the same manner as they use DSE_VERSION