CBQE-2370: restore magic variable resolution in st

templates that use magic variables such as $str10, $boo, $int5 are not being resolved with respective values.
this code existed in previous implementation of system test but was lost as underlying loaders was changed and needs to be reimplemented for use with current loader.

Change-Id: Iee13cc52dac11df75185ac2296dca4db02bd53cd
Reviewed-on: http://review.couchbase.org/40381
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Tommie McAfee <tommie@couchbase.com>