Update integration tests password

The new gate job to run the integration tests is currently failing
because the password defined in horizon.conf does not match the
password created for devstack on the gate. It should be updated to
"secretadmin".

Change-Id: I71e37dcc78c2c6d5d7666a6aa466afa84e91629d
Closes-Bug: #1392269
Merge "Update integration tests password"