We're adding 2 services that are responsible for executing db purge and
archive_deleted_rows. Services will be deployed by default, but left
stopped/disabled. This way we allow deployers to enable/disable
feature by changing value of nova_archive/purge_deleted.
Otherwise, when variables set to true once, setting them to false won't
lead to stopoing of DB trimming and that would need to be done manualy.
Change-Id: I9f110f663fae71f5f3c01c6d09e6d1302d517466
efe64725e1