Ambari 1.7 has not been released. But you can follow the project page and they have info on the repo for a nightly build:
https://cwiki.apache.org/confluence/display/AMBARI/Quick+Start+Guide
# to test the 1.7.0 branch build – updated nightly
wget -O /etc/yum.repos.d/ambari.repo http://s3.amazonaws.com/dev.hortonworks.com/ambari/centos6/1.x/latest/1.7.0/ambari.repo