Bump swap to 3GB in linuxbridge and openvswitch jobs

Since [1] linuxbridge and openvswitch jobs are randomly
failing as swap gets fully utilized and processes are
getting oom-killed, to avoid this increasing swap size
to 3GB.

[1] https://review.opendev.org/c/openstack/devstack/+/924094

Closes-Bug: #2073582
Change-Id: I3889beb4c3427dd14ba1d1cddc9027b366268c1b
1 file changed