Refactor server password metadata to avoid direct db usage

This refactors the set_password() method in the server_password
API extension to merely format the metadata and return it, leaving
the job of updating the database to the caller. In the API extension
case, the update can be done immediately against the database as
before. In the case of the metadata API handler and the xenapi virt
driver making the call, conductor can be used to update the instance's
system_metadata, thereby avoiding a direct database access.

Related to blueprint no-db-compute

Change-Id: I0563feaa97d768a96f950c148b1dbf51c356c4ac
Merge "Refactor server password metadata to avoid direct db usage"