Avoid failing at import time if krbV module is missing

Several parts of Koji already handled this absence gracefully. This change
extends that behavior to all places where krbV is imported.

util/koji-gc

@@ -9,7 +9,7 @@
 try:
     import krbV
 except ImportError:  # pragma: no cover
-    pass
+    krbV = None
 import koji
 from koji.util import LazyDict, LazyValue
 import koji.policy
@@ -345,7 +345,7 @@ def ensure_connection(session):
         warn(_("WARNING: The server is at API version %d and the client is at %d" % (ret, koji.API_VERSION)))
 
 def has_krb_creds():
-    if 'krbV' not in sys.modules:
+    if krbV is None or 'krbV' not in sys.modules:
         return False
     try:
         ctx = krbV.default_context()

util/koji-shadow

@@ -25,7 +25,7 @@
 try:
     import krbV
 except ImportError:  # pragma: no cover
-    pass
+    krbV = None
 import koji
 import ConfigParser
 import fnmatch
@@ -320,7 +320,7 @@ def activate_session(session):
     elif options.user:
         #authenticate using user/password
         session.login()
-    elif 'krbV' in sys.modules:
+    elif krbV and 'krbV' in sys.modules:
         try:
             if options.keytab and options.principal:
                 session.krb_login(principal=options.principal, keytab=options.keytab, proxyuser=options.runas)