The subroles (setup/tree_retrieval/indexing) are merged to the single role: lxr

This is a refactoring effort to handle multiple-platforms easier.  Now, vars/ and defaults/ in Lxr role are used.  The site-wide group_vars/ can overwrite the variables for customizations.
A few English sentences are revised.
Auto-generated lxr_tag_name is implemented.

When git repo is used, the playbook is now capable of generating lxr_tag_name out of the commit date and the short hash.  If the user define lxr_tag_name explicitly in group_vars/, the defined value overrides the default behavior.
Extra {{ and }} are removed where when: conditionals are used.
Missing tags are added.
Some cosmetic changes.