aptrepo: make conf/incoming configurable parameter
Because we have 2 use cases for aptrepo, carbon (apt.wm) and bromine
(releases.wm) with different config for incoming uploads, we need
to make this configurable.
Adds a parameter incomingconf to set to a template name
that will have the content of ./reprepro/conf/incoming.
Bug:T132757
Change-Id: Ibaf755e1e92f8cf3907665ea86bf054bc96d0da1