Fix get_distribution for Alpine Linux (#17224)

To override a generic class that is subclassed based on platform, the
subclass must define platform and distribution.

The load_platform_subclass() calls the get_platform() and
get_distribution() methods to detect the platform and the distribution.

On Alpine Linux, get_distribution() method returns None and it is not
possible to have different implementations based on detected platform.

lib/ansible/module_utils/basic.py

@@ -218,7 +218,7 @@ def get_distribution():
     ''' return the distribution name '''
     if platform.system() == 'Linux':
         try:
-            supported_dists = platform._supported_dists + ('arch',)
+            supported_dists = platform._supported_dists + ('arch','alpine')
             distribution = platform.linux_distribution(supported_dists=supported_dists)[0].capitalize()
             if not distribution and os.path.isfile('/etc/system-release'):
                 distribution = platform.linux_distribution(supported_dists=['system'])[0].capitalize()