JIRA should not try to change passwords for users that belong to Read Only user directories.

Initially raised in EAP feedback:
http://jira.atlassian.com/browse/JRA-23033?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=231531#comment-231531

There are a couple of scenarios where we know it is not possible for a user's password to be changed from within JIRA:

• When the user comes from Read Only LDAP directory
• When the user lives in "Internal Directory with LDAP Auth"
• Can fail under a Crowd Directory - but we don't know beforehand

And we can change passwords in the following places:

• Admin can change a user's password
• User can change their own password
• User can make a "reset password" request when they forget password.
  This one is a little tricky and probably requires an amended email message.

When this work is completed, I think we can remove the global "External Password Management" setting that is a hangover of OSUser delegated LDAP.