Allow multiple distribution destinations for virtual repositories

A virtual repository aggregates one or many local repositories of a specific type. All artifacts from all repositories can be read through the virtual repository as expected. However, none or only one repository can be chosen as a distribution repository. This can be confusing as writing to a virtual repository can end up writing to some specific local repository only specified in Artifactory's backend but not from the client.

As an user, I expect to be able to read and write to multiple repositories through the virtual repository. For example, if virtual repository A contains local repositories A, B and C, I should be able to write to all A, B and C local repositories from the virtual repository A and not only one of them (for example C).