Tag to be used with the 21.X branches of atlas/athena.
Update(s) in this tag wrt. 1.0.27:
- Updated the Simage build to succeed on CentOS 7;
- Updated the UUID external handling of AtlasLCG, AthenaExternals, AthDerivationExternals and yampl, to be compatible with CentOS 7;
* Including the update of yampl from version "4a91d89" to "46f7a48".