Add EL7 < 7.5 compatibility warnings to XRootD and OASIS docs

Add a bug admonition (https://squidfunk.github.io/mkdocs-material/extensions/admonition/#bug) with the title "EL7 version compatibility" to the top of the following documents:

• https://opensciencegrid.org/docs/worker-node/install-cvmfs/

• https://opensciencegrid.org/docs/data/xrootd/install-standalone/

• https://opensciencegrid.org/docs/data/xrootd/install-storage-element/

• https://opensciencegrid.org/docs/data/xrootd/xrootd-authorization/

• https://opensciencegrid.org/docs/data/xrootd/install-client/

The CVMFS incompatibility is due to an old version of selinux-policy while the XRootD incompatibility is due to issues with `xrootd-lcmaps`.

We can hold off on addressing this ticket until the doc focus on 2017-07-25