improve NamedContextFactory.getInstance() perfomance

[spencergibb]

for NamedContextFactory.getInstance(String name, Class<T> type) there's no reason to call BeanFactoryUtils.beanNamesForTypeIncludingAncestors(). Just catch NoSuchBeanDefinitionException and return null.

See spring-cloud/spring-cloud-gateway#1941

[spencergibb]

/cc @lkolisko