Generalized files selection using files_switch

... and related documentation in README.
Fix documentation
Fixed repo definition to work in non Debian OS

The idea is to maintain the structure ready just in case someone wants
to define a pkgrepo for other os_family values.
Now if the os_family is not defined nothing breaks (hopefully) :-)
Merge branch 'master' of github.com:moreda/zabbix-saltstack-formula