Update version to 1.5.1-1-GA
Update release notes for 1.5.1

Use relative doc links for release notes in docs

Add previous versions to version history

Add date to version history title in TOC
Remove duplicate unused templates
Fix out of date template

The rundeck-config.properties.template file was not updated in the right
location.  Removed old template files and updated correct template
Fix incorrect configuration of plugin manager

If rdeck.config was not set in rundeck config file, then the plugin
manager used for Notification plugins was initialized with an invalid
plugin libext path, and would create a '[:]' directory.

* Require either config file or system property definition of 'rdeck.base'
inside the spring resources script, otherwise bean definitions used by
the notification plugin system will not be created.
* update NotificationService to handle a missing plugin registry
Add rdeck.base to default config for rpm/deb
init scripts passing wrong basedir to RunServer

Use of /etc/rundeck instead of /var/lib/rundeck as the basedir
for deb/rpm init scripts.
Fix typo for dir in init script
Fix param parsing single notification plugin

If only one Notification plugin was installed, the input
form parameters were not handled correctly.

* use flatten convert list or single string into a list
Fix NPE for notif plugin without config properties

If a notification plugin has no config properties, then it will
fail when triggered due to NPE

* avoid error when config input is null