Previously, spring.groovy.template.configuration.* was mapped onto both
GroovyTemplateProperties.configuration and GroovyMarkupConfigurer. The
former being a Map and the latter being specific type with getters and
setters. This clash caused problems with the IDE support.

GroovyTemplateProperties.configuration appeared to be dead code so it
was removed in 326bdf29. Unfortunately this broke the use of
spring.groovy.template.configuration.* properties as
GroovyTemplateProperties uses a prefix of spring.groovy.template and it
no longer had a configuration property.

This commit addresses the problem by updating GroovyTemplateProperties
to ignore unknown fields. This allows
spring.groovy.template.configuration.* properties to be used and bound
to GroovyMarkupConfigurer without reintroducing the clash which prompted
the initial change.

Closes gh-3198