Fixes #23498 - run with mongo 3.4 from scl

[mbacovsky]

To get version of the db server (may be remote) we assume the
'mongo' command is always available. If there is only mongo from SCL
present the mongo client has to be called from within a scl.

With current state of things it is safe to assume there is at least
some mongo client installed. For the future We will need to figure out
how to install a correct version of the client.

[iNecas]

ACK codewise. alda519 mind testing to check it fixes the issue for you?

[alda519]

Sure.
And it does. But there is one more problem - if the mongo service is down, then server_version cannot connect to it to get the version.
What about simplifying it, and use @core = available_core directly? But I am not sure if we can rely on that the server is the same version as client and what happens to mongo 2.6 packages after upgrade to 3.4)

[mbacovsky]

@alda519 that is my plan. I'll try to guess version from the running server with fallback to the installed packages (aka available_core) as you suggest. The only issue to solve is that there could be more mongo versions installed and we can not tell what is used when the service is down. I'll update this PR with the fix.

[mbacovsky]

@alda519 could you please re-test?

[alda519]

Yup, now it works when mongo is down. Perfect.

[iNecas]

One last thing: rubocop is not happy. Going to merge after that's resolved.

[mbacovsky]

@iNecas rubocop seems satisfied now.

[iNecas]

Thanks @mbacovsky