Improve combination rule validation

Combination rule alarms should contain at least two different alarms
in the alarm_ids list. This patch adds a check to the validate method
of AlarmCombinationRule and raise ClientSideError, if the condition
fails.

Closes-Bug: #1305519

Change-Id: Ic5968d6ecf5781070cab39274131e605619e264a
Describe storage backends in the collector installation guide

Move information about the storage backends to the collector
section and add info about their usage.

Change-Id: I979a458c91f5dee486799f201b8f4d75a4163782
Merge "Describe storage backends in the collector installation guide"
Merge "Improve combination rule validation"