CI: new Python classes for SSH & Local execution

The existing SSHConnection class is ... not good:

• it does not give the return code
• it typically scans for a single & before returning (this can lead to false-positives, e.g., it did in !1806 (merged) where a & appeared in a diff)
• it assumes password-based SSH auth
• there is no corresponding "local execution" class with the same interface
• copyin/copyout spawn new processes
• ... probably more

I add two new classes LocalCmd and RemoteCmd. They have the same interface. Results of a call are returned in the Python subprocess class format. We can now check for return codes, and stuff with & cannot happen. RemoteCmd has no means for a password, and assumes certificate-based SSH. If the return-code of any command is non-null, it will print a warning and the command output (this can be suppressed). For the moment, only the cluster-build and (RHEL) and image-build (Ubuntu) builders use it (more to follow later, I don't want to change everything at once). There interface is somewhat backwards-compatible to SSHConnection, both have a command call.

Also, the tags in the cluster images follow the branch-commitId[1:8] pattern of the Ubuntu CI.