Make ${OS_NODE_CNT} a suite variable

Make ${OS_NODE_CNT} a suite variable

Without making it a suite variable, it seems the changed
value is only local to that keyword, and outside of that
keyword it will remain the value as assigned in the
above *** Variables *** section, which is ${1}

example of the value getting changed to 3:
https://logs.opendaylight.org/sandbox/vex-yul-odl-jenkins-2/jamo-netvirt-csit-1node-0cmb-1ctl-2cmp-apex-queens-gate-snat-conntrack-oxygen/42/robot-plugin/log_full.html.gz#s1-k1-k1-k4-k2-k4

two keywords later in that same job, you can see
it's incorrectly back to 1:
https://logs.opendaylight.org/sandbox/vex-yul-odl-jenkins-2/jamo-netvirt-csit-1node-0cmb-1ctl-2cmp-apex-queens-gate-snat-conntrack-oxygen/42/robot-plugin/log_full.html.gz#s1-k1-k1-k4-k2-k6-k2

Change-Id: I6e8cc9eb73acc8efef8a61342546d2de1205e4bb
Signed-off-by: Jamo Luhrsen <jluhrsen@redhat.com>