Link sub_alarm to sub_alarm_expression

Linking sub_alarm to sub_alarm_expression allows the Threshhold engine
to change SubAlarms when the Alarm Definition is changed.

Also, increase size of dimension_name and value in the
sub_alarm_definition_dimension table to match what the API accepts

Remove the primary key from the sub_alarm_definition_dimension
table because increasing the size of dimension_name exceeds the
size thet mysql will accept for an index. This key isn't really
needed

These are the same changes that were made to cookbook-monasca-schema