Update defaults/guides with details about new nfs_mounts usage

[martbhell]

These should be updated after the new version of the nfs_mount role is available in master branch.

• The PR: https://github.com/CSC-IT-Center-for-Science/ansible-role-nfs_mount/pull/3
• The guide: https://confluence.csc.fi/display/FGCI/FGCI+Cluster+deployment+and+installation+guide
• The defaults: https://github.com/CSC-IT-Center-for-Science/fgci-ansible/tree/master/examples/group_vars

[martbhell]

Defaults updated. Some documentation in the release notes.