Clean up session in db.sqla.api.migration_* methods

Remove session parameter from migration_get():
Add _migration_get() that contains same logic that old migration_get(),
for internal usage, and new migration_get() that doesn't contains session
parameter.

Remove session parameters from:
migration_get_unconfirmed_by_dest_compute()
migration_get_in_progress_by_host_and_node()

They are never called with session parameter.

blueprint db-session-cleanup

Change-Id: I3664c78b6aba4c0640903d0f935bc261d5d2129b
Merge "Clean up session in db.sqla.api.migration_* methods"