A few tweaks that might improve performance on startup

... or couldn't hurt anyway.

1. Extends the definition of a web application for @ConditionalOnWebapp
so that a StandardEnvironment can be used (cutting out JNDI failures
for Environment properties)

2. Doesn't bother using StandardServletEnvironment in integration tests

3. Make the NON_ENUMERABLE_ENUMERABLES in PropertySourcesPropertyValues
static so they only get initialized once (not a huge issue at all)