Move Echo configuration into BeforeCreateEchoEvent hook

Move configuration of $wgEchoNotifications and
$wgEchoNotificationCategories into a function to be invoked by the
BeforeCreateEchoEvent hook. Also rename efEchoGetDefaultNotifiedUsers to
efOpenStackGetDefaultNotifiedUsers for more consistent pseudo
namespacing and group all Echo configuration together in the source
file.

Change-Id: I68cf444dacf37d17e38b968edcfcc88193d46681
Merge "Move Echo configuration into BeforeCreateEchoEvent hook"