* CP-27819: Manually setting the ToolsVersion to 15.0 (VS2017) in all .csproj files for consistency and to attempt to enforce consistency of dev environments with the explicitness of the msbuild scripts as used in the build environments.
Auto updated as many of the .sln and .csproj files as VS2017 was willing to do https://docs.microsoft.com/en-us/visualstudio/ide/reference/upgrade-devenv-exe?view=vs-2017 and included changes to the gitignore to ensure that the update report and backup files are not included in the repo.
* CP-27819: Tidying up by removing FileUpgradeFlags, OldToolsVersion and UpgradeBackupLocation nodes from the .csproj files as they are produced as hints for how to revert project upgrades for which we can use version control.
* CP-27819: Removing redundant "ManifestCertificateThumbprint" and "ManifestTimestampUrl" nodes from project files "mk\sign.bat" has been used for this purpose for some time now.
* CP-27819: Updated readme to reflect that VS2017 is now in use.
Signed-off-by: Aaron Robson <aaron.robson@citrix.com>
