Increase aaa-release-merge build timeout

release-merge job now seems to be correctly waiting for repo promotion,
hence the default 15 minutes do not suffice. Bump to 1 hour as a first
cut (which is 6x the time it takes to create and test a release).

Change-Id: Ie74ddc1f9931d27a17b53424c000539bd482c342
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>