Plan branch should be able to override multiple source repositories

The plan branch feature currently can override only the default source repository. To make the plan branch useful in case there are multiple source repositories (that have the same svn trunk location as parent directory), the plan branch feature should be extended to substitute the trunk URL for the branch URL in all defined source repositories.

Details on our use case:
In our build plan, we have defined multiple source repositories, each of them is a subdirectory of the Trunk. This way we can take advantage of jobs doing building in parallel, and each job does an svn checkout only of what's needed to build. (The SVN tree contains more stuff than what is needed for a build.)

Our svn structure and corresponding source repo's look like:
– trunk
---- DB (Bamboo repo 1 = SVNURL/trunk/DB)
---- OSB (Bamboo repo 2 = SVNURL/trunk/OSB)
---- SCA (Bamboo repo 3 = SVNURL/trunk/SCA)

Now we want to use the plan braching feature, in order to do a build on a previous release. In SVN we have a branches for each major version of our product. To execute the build correctly, all source locations must now point to the branch location. In case the branch is located at SVNURL/branches/R1.0, the source repositories must become the following:
Bamboo repo 1: SVNURL/branches/R1.0/DB)
Bamboo repo 2: SVNURL/branches/R1.0/OSB)
Bamboo repo 3: SVNURL/branches/R1.0/SCA)