Resolve issues raised by LukeM's review

* Remove duplicated code.
* Clean up conditionals.
* Shorten fail message.

tasks/main.yml

 ---
+- name: Check OS distribution and version
+  set_fact:
+    distro_ok: "{{ ansible_distribution == 'CentOS' }}"
+    version_ok: "{{ ansible_distribution_major_version in ['6', '7'] }}"
+
 - name: Ensure an error is raised on unsupported OS
   fail:
-      msg: "Currently Software Collections are only supported on CentOS 6 and 7. Do not install on other setups - see role README"
-  when: ansible_distribution != "CentOS" or (ansible_distribution_major_version != "6" and ansible_distribution_major_version != "7")
-
-- name: Add Software Collections on CentOS 6
-  import_tasks: scl-centos6.yml
-  when: ansible_distribution == "CentOS" and ansible_distribution_major_version == "6"
+    msg: "Only CentOS 6 and 7 are supported"
+  when: not (distro_ok and version_ok)
 
-- name: Add Software Collections on CentOS 7
-  import_tasks: scl-centos7.yml
-  when: ansible_distribution == "CentOS" and ansible_distribution_major_version == "7"
+- name: Add Software Collections for CentOS
+  yum:
+    name: centos-release-scl
+    state: present
+  when:
+    - distro_ok
+    - version_ok

tasks/scl-centos6.yml

----
-- name: Add Software Collections for CentOS 6
-  yum:
-      name: centos-release-scl
-      state: present

tasks/scl-centos7.yml

----
-- name: Add Software Collections for CentOS 7
-  yum:
-      name: centos-release-scl
-      state: present