Use Priority instead of Severity

[frantz45]

When we created Logging Alert plugin we created the field Severity because there was no equivalent in Graylog. But now there is the Priority field. Priority has only 3 levels, and Severity 4, but it may be enough.

[c8y3]

This is a breaking change. It seems to me that it implies a migration step (exporting/importing all rules). The mapping from current severity levels to priorities needs to be defined in order to have the import accept ancient rules.

[VincentD06]

The mapping for this feature is :

Severity	Priority
INFO	LOW
FAIBLE	LOW
MOYENNE	NORMAL
HAUTE	HIGH