Allow Tomcat context root redirect to be configured via the environment

This commit adds a new property, server.tomcat.redirect-context-root,
that can be used to configure the Tomcat Context’s 
mapperContextRootRedirectEnabled property. The default is to not apply
any configuration and, therefore, to use Tomcat’s default of true.

Closes gh-6248

spring-boot-docs/src/main/asciidoc/appendix-application-properties.adoc

@@ -214,6 +214,7 @@ content into your application; rather pick only the properties that you need.
 	server.tomcat.port-header=X-Forwarded-Port # Name of the HTTP header used to override the original port value.
 	server.tomcat.protocol-header= # Header that holds the incoming protocol, usually named "X-Forwarded-Proto".
 	server.tomcat.protocol-header-https-value=https # Value of the protocol header that indicates that the incoming request uses SSL.
+	server.tomcat.redirect-context-root= # Whether requests to the context root should be redirected by appending a / to the path.
 	server.tomcat.remote-ip-header= # Name of the http header from which the remote ip is extracted. For instance `X-FORWARDED-FOR`
 	server.tomcat.uri-encoding=UTF-8 # Character encoding to use to decode the URI.
 	server.undertow.accesslog.dir= # Undertow access log directory.