Change master->primary/leader/member (#2541)

keep as much backward compatibility as possible.

Following changes were made:
1. All internal checks are performed as `role in ('master', 'primary')`
2. All internal variables/functions/methods are renamed
3. `GET /metrics` endpoint returns `patroni_primary` in addition to `patroni_master`.
4. Logs are changed to use leader/primary/member/remote depending on the context
5. Unit-tests are using only role = 'primary' instead of 'master' to verify that 1 works.
6. patronictl still supports old syntax, but also accepts `--leader` and `--primary`.
7. `master_(start|stop)_timeout` is automatically translated to `primary_(start|stop)_timeout` if the last one is not set.
8. updated the documentation and some examples

Future plan: in the next major release switch role name from `master` to `primary` and maybe drop `master` altogether.
The Kubernetes implementation will require more work and keep two labels in parallel. Label values should probably be configurable as described in https://github.com/zalando/patroni/issues/2495.

features/environment.py

@@ -905,7 +905,7 @@ class PatroniPoolController(object):
         }
         self.start(name, custom_config=custom_config)
 
-    def bootstrap_from_backup_no_master(self, name, cluster_name):
+    def bootstrap_from_backup_no_leader(self, name, cluster_name):
         custom_config = {
             'scope': cluster_name,
             'postgresql': {
@@ -914,11 +914,11 @@ class PatroniPoolController(object):
                                         '--dirname {} --filename %f --pathname %p').format(
                                         os.path.join(self.patroni_path, 'data', 'wal_archive').replace('\\', '/'))
                 },
-                'create_replica_methods': ['no_master_bootstrap'],
-                'no_master_bootstrap': {
+                'create_replica_methods': ['no_leader_bootstrap'],
+                'no_leader_bootstrap': {
                     'command': (self.BACKUP_RESTORE_SCRIPT + ' --sourcedir=' +
                                 os.path.join(self.patroni_path, 'data', 'basebackup').replace('\\', '/')),
-                    'no_master': '1'
+                    'no_leader': '1'
                 }
             }
         }