[7.1] Security Configuration Guide: Add FIPS mode notes

Add to migration guide this information:

• Default behaviour change (in EAP 7.1 running in FIPS mode will require additional configuration)
• Taking a previously working FIPS configuration from EAP 7.0 works in FIPS mode on a JBoss EAP 7.1 installation

This documentation issue is caused by discussion on:
https://issues.jboss.org/browse/JBEAP-6106
https://issues.jboss.org/browse/EAP7-274, particularly comment
https://issues.jboss.org/browse/EAP7-274?focusedCommentId=13319860&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-13319860

This issue does not need clone for 7.0.0.GA. It applies only to EAP 7.1 migration guide.