Goal

• Unify names of variables within the leapp collection

Acceptance criteria

• Variables to be defined/changed by users should all locate in roles' defaults/main.yml, this is Ansible way to distinguish between variables used by the role internal (located in vars/), and variables accessibly by the user in defaults/. 
• Prefix all variables in defaults/main.yml will `leapp_`. In the case when there is a conflict with the same variable name between roles, prefix variables with `leapp_rolename_`. Note that such a case might not be a conflict and it's intentional that roles have same variables, in this case keeping only `leapp_` is good.
• Prefix variables in vars/ with `_leapp` - double underscore before the variable indicates that it is to be used internally within the role only.
• Add tasks that create aliases for new varnames in the case when user defines an old variable name. Example in mssql: https://github.com/linux-system-roles/mssql/blob/main/tasks/main.yml#L13-L26
  This is required to keep backward compatibility for users who used leapp collection previously so that they don't need to rename all their variables.
  Print a message saying that the old variable is deprecated and will be removed in a future release.