email notification: group failures by the starting build

[mwasilew]

Imported from https://projects.linaro.org/browse/QA-1671 Currently each failure is listed with the 'starting point'. This might make the lines with failures listed long. The goal of this ticket is to group the failures by the 'starting point'. For example:

{code} failed since version abcdef1 testsuite_foo/test1 testsuite_foo/test2

failed since version ghijk1 testsuite_bar/test2 {code}

[chaws]

Closing it due to the same reason of https://github.com/Linaro/squad/issues/180. I think the list can be written in the email template. Please re-open if I'm wrong