More flexible handling of `upload_repo`

I have some problems with running the new upload handling in ConanMultiPackager using our private conan registry.

First problem, I already have a conan remote entry with the same address as the one specified in the upload parameter. Which then gives the following error:

ERROR: Remote 'foo' already exists with same URL

Maybe the upload parameter should be able to specify an existing remote entry if the build environment already has this set up?

If I remove this entry this entry before running the builder I can upload the package. But, if I run the build script a second time the packager.py tries to add the upload_repoagain, even though it already exist.

ERROR: Remote 'upload_repo' already exists in remotes (use update to modify)

I do not see where this is removed from the list of remotes?

[ghost]

I think it’s still doing it https://travis-ci.org/dimi309/conan-packages/jobs/244790536

[ghost]

I can test tonight.