Role will now always attempt to unmask on role run

add variable 'firewall_disable_conflicting_services' to enable the
disabling of conflicting services
- Set to false by default
  - Requires that services are enumerated on managed nodes, which can
    introduce potentially unnecessary runtime overhead

Update README to document the following behavior of the system role:
- linux-system-roles.firewall will attempt to install, unmask, and enable firewalld
- linux-system-roles.firewall can attempt to disable directly conflicting services to firewalld
  - and that is enabled by setting the variable 'firewall_disable_conflicting_services' to true
  - list of conflicting services present in vars/main.yml
test cases for these changes in tests/tests_default.yml

Addresses GitHub Issues: linux-system-roles#103, linux-system-roles#136

List out conflicting services checked for in README, move some instruction in the introduction down to relevant variable.

Return tests_default.yml to its original state

Move tests for unmasking firewalld and removing conflicting services to tests_startup_conflicts.yml