Revert testplan change and add custom csit types

Revert testplan change and add custom csit types

Revert testplan change caused by
Iddb9da21f490670255f93f8aa730063027a668b8 and add a custom
csit template to allow projects to override testplan at the
project level.

Nesting variables in this file like what was done for testplan
caused every template in releng/builder to require all of
the same variables as CSIT jobs which is a problem because
a job-template that doesn't care about certain variables should
not be required to suddenly require them.

Change-Id: I895b6a41276a5a9434649948e9332f31ae9d3676
Signed-off-by: Thanh Ha <thanh.ha@linuxfoundation.org>