gerrit: Use hyphenated key names.

Hyphenated strings are now standard in Jenkins Job Builder.

Gerrit Trigger Plugin support used camelCase keys for its
configuration, and is updated to use hyphenated strings.

Old configuration style is still supported: if old key names
are found, they are converted to new style key names.

Change-Id: I3b1968f14029b69497ee543b44070db7d9c3448c
Reviewed-on: https://review.openstack.org/24730
Reviewed-by: James E. Blair <corvus@inaugust.com>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Anita Kuno <akuno@lavabit.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins