Improve error message if realms are in old format

[tvernum]

If a 7.0 node is started with a 6.x style realm config (see #30241)
then the error message is something like this:

Caused by: org.elasticsearch.common.settings.SettingsException: Failed to get setting group for [] setting prefix and setting [bind_dn] because of a missing '.'
	at org.elasticsearch.common.settings.Settings.getGroupsInternal(Settings.java:494) ~[elasticsearch-7.0.0-SNAPSHOT.jar:7.0.0-SNAPSHOT]
	at org.elasticsearch.common.settings.Settings.getAsGroups(Settings.java:510) ~[elasticsearch-7.0.0-SNAPSHOT.jar:7.0.0-SNAPSHOT]
	at org.elasticsearch.common.settings.Settings.getAsGroups(Settings.java:506) ~[elasticsearch-7.0.0-SNAPSHOT.jar:7.0.0-SNAPSHOT]
	at org.elasticsearch.xpack.core.ssl.SSLService.lambda$getRealmsSSLSettings$4(SSLService.java:623) ~[?:?]
	at java.util.HashMap.forEach(HashMap.java:1336) ~[?:?]

We should fail with something more actionable.

[elasticmachine]

Pinging @elastic/es-security