do not create SSH connection on import

Prevent an SSH connection from automatically being established when module `robottelo.common.ssh` is imported. Instead, generate connections as needed. __This may significantly reduce the speed of the test suite.__

Automatically close SSH connections, just like one would do with a file handle.

Fix a broken method call.
@omaciel Yes, the last line is executed. You can check for yourself by placing a "print" statement after the `conf._properties = backup` statement.

Test methods are executed in full, and it is possible to take advantage of this behavior and do things like place multiple assertions in a single test method. However, placing multiple assertions in a single test function is bad practice.