LCG nightly support, master branch (2017.04.12.)

Taking Marcin's implementation as a starting point, updated the AtlasLCG code to allow the user to override the release number requested by a project by setting the $LCG_NIGHTLY environment variable. So that LCG nightlies wouldn't need a separate branch in the repository. And so that we could set a different day of the week for the nightlies from the build script itself.

At the same time updated the code to look for LCG releases on CVMFS instead of AFS by default, if the user didn't specify anything explicitly.