Be more defensive when checking Vault

When relations are departing during a unit removal, the relation
to Vault can loose enough data that the vault-relation-departed hook
raises an exception. The checks for Vault readiness should be more
defensive to ensure that charms can successfully depart their relations

Depends-On https://github.com/juju/charm-helpers/pull/445
Closes-Bug: #1868282

Change-Id: I0b6226f0f3500aef7304f8e8b38d06daebfd0c20

hooks/charmhelpers/contrib/openstack/vaultlocker.py

@@ -140,9 +140,16 @@ def vault_relation_complete(backend=None):
     :ptype backend: string
     :returns: whether the relation to vault is complete
     :rtype: bool"""
+    try:
+        import hvac
+    except ImportError:
+        return False
+    try:
         vault_kv = VaultKVContext(secret_backend=backend or VAULTLOCKER_BACKEND)
         vault_kv()
         return vault_kv.complete
+    except hvac.exceptions.InvalidRequest:
+        return False
 
 
 # TODO: contrib a high level unwrap method to hvac that works