Fix #3 w/ CentOS 7.7+ compatibility for gsiam.py

Python functions that Yum expects in plugins changed somewhere between
CentOS 7.4 and 7.7, and on 7.7, Yum is unable to use `gsiam` to parse
`gs://` URLs:

```
[root@f5645f8162d5 /]# yum repolist
Loaded plugins: fastestmirror, gsiam, ovl
Loading mirror speeds from cached hostfile
YumRepo Error: All mirror URLs are not using ftp, http[s] or file.
 Eg. gs://example-bucket/path-to-repo/
```

This patch maintains CentOS 7.4 compatibility and adds CentOS 7.7 and
7.8 compatibility, as well.

VERSION

@@ -1,2 +1,2 @@
 NAME=yum-plugin-gs-iam
-VERSION=1.0.0
+VERSION=1.1.0

gsiam.py

@@ -15,7 +15,7 @@
 URL_SCHEME = 'gs://'
 
 __all__ = ['requires_api_version', 'plugin_type', 'CONDUIT',
-           'config_hook', 'prereposetup_hook']
+           'config_hook', 'init_hook', 'prereposetup_hook']
 
 requires_api_version = '2.5'
 plugin_type = yum.plugins.TYPE_CORE
@@ -54,7 +54,7 @@ def replace_repo(repos, repo):
     repos.add(GCSRepository(repo.id, repo))
 
 
-def prereposetup_hook(conduit):
+def init_hook(conduit):
   """Plugin initialization hook. Setup the GCS repositories."""
   repos = conduit.getRepos()
   for repo in repos.listEnabled():
@@ -66,6 +66,11 @@ def prereposetup_hook(conduit):
       replace_repo(repos, repo)
 
 
+def prereposetup_hook(conduit):
+  """Maintain compatibility with Yum on older CentOS (< 7.7.1908) releases."""
+  return init_hook(conduit)
+
+
 class GCSRepository(YumRepository):
 
   def __init__(self, repoid, repo):
@@ -82,7 +87,7 @@ def __init__(self, repoid, repo):
 
     if repo.google_application_credentials:
       os.environ[environment_vars.CREDENTIALS] = repo.google_application_credentials
-    
+
     self.bucket = bucket
     self.base_path = path
     self.name = repo.name