Externals Update (CentOS 7 Fixes), 21.0-mc16a branch (2020.10.05.)

Updated all projects to atlasexternals-1.0.27.2 to fix the build issues of Athena-21.0-mc16a on x86_64-centos7-gcc62-opt, as described in ATLINFR-3741.

The full list of changes wrt. atlasexternals-1.0.27 is (atlasexternals@1.0.27...1.0.27.2):

• Updated the Simage build to succeed on CentOS 7 (cherry-picking an ancient fix from @rbianchi);
• Updated the UUID external handling of AtlasLCG, AthenaExternals, AthDerivationExternals and yampl, to be compatible with CentOS 7 (cherry-picking fixes made for the 21.0 branch recently);
  • Including the update of yampl from version 4a91d89 to 46f7a48;
• Updated atlas_setup_cpack() to create a functional setup for building RPMs with the latest OS updates.

With all these updates I was able to build Athena-21.0-mc16a out of this branch as-is. But let's see if the CI can do that as well...