Fix Pull Request update in UpdateSubmodules commmand

Edit the Pull Request body with new commit message rather than adding
a comment.
Separate remote addition from PR opening in Sandbox library
Review UpdateSubmodules integration tests

- Test wether pr is opened with --push option
- Add test for --no-pr option
- Add test for multiple run of `scc update-submodules`
Expand Merge integration tests
Rename TestUpdateSubmodule class
Add tests for --remote optional argument in TestMerge
Merge pull request #94 from sbesson/updatesubmodule_prupdate

Update-submodules PR update