Revert daily email subject back to original

For now the daily email subject needs to be reverted back to
what it was before change I20beb103b0779cf0c8b201c5117885be10912a83.
A new variable needs to be introduced in Zuul first before
a unique subject line can be implemented for the daily jobs.

Change-Id: Ib9fd429e3ea6acd0c7ab2364f18a303214684619