Fix setting backdoor port in service start

Previously if self.manager wasn't defined for a Service object but a
backdoor_port was specified then that particular service would not
start because it would try to set the backdoor port in an object that
didn't exist. This commit fixes setting the backdoor port only if
manager is defined when calling start().

Closes-Bug: #1243260

Change-Id: Ib0e709cfd0e299a3e010450b7eba6e92bc108e4e
(cherry picked from commit e5c96505c0b4eac51c0925dcb691f0a1699216ce)
Merge "Fix setting backdoor port in service start" into stable/havana