Fixes behavior of ipavault when no user, service or shared is given.

IPA CLI allows the creation of vaults without specifying user, service or a
shared vault, defaulting to create a user vault for the `admin` user. The
vault module, required that one of user, service or shared was explicitly
provided, and this patch makes the module behave like the CLI command.

Tests were added to reflect this change.

plugins/modules/ipavault.py

@@ -402,7 +402,6 @@ def main():
         ),
         supports_check_mode=True,
         mutually_exclusive=[['username', 'service', 'shared']],
-        required_one_of=[['username', 'service', 'shared']]
     )
 
     ansible_module._ansible_debug = True