CBQE-683: allow magic values query params

Random values will be generated for startkey/endkey.
This is useful when auto query was enabled or for
running queries against non system-test data.

python cbsystest.py run query --cluster default --ddoc ddoc1 --view view1 --queries_per_sec 50 --template default --startkey '$int4' --endkey '$int4'

Change-Id: I80bdfc92e0d2adfd5b043467d80118e658d41427
Reviewed-on: http://review.couchbase.org/25000
Tested-by: Tommie McAfee <tommie@couchbase.com>
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Tommie McAfee <tommie@couchbase.com>