Refactor db.service_destroy and db.service_update methods

Remove session parameter from db.service_get method

Optimize db.service_destroy:
Was:
1 request with join to get service with compute_node
1 request to delete service
1(0) to delete compute_node
Now:
1 request (without join) to delete service
1 request to delete compute_node

Optimize db.service_update:
Remove unused join.

Fixes bug 1157442

blueprint db-session-cleanup

Change-Id: Ifcc8fdebfc95daec736464cbc9e16301116540aa
Merge "Refactor db.service_destroy and db.service_update methods"