System and partition initialisation are random

[vkarak]

This is due to the fact that both systems and partitions are defined as dictionaries. This has several repercussions:

• No specific order that the regression goes through the partitions during the phase of test execution.
• Hostname matching rules must be unique. No last resort, catch-all pattern may be placed, since this may be tried earlier and lead to false system detection.

With Python 3.7 normal dictionaries will become ordered, but it's better not to rely on that.

Internal issue: https://madra.cscs.ch/scs/reframe/issues/234