qa: ignore laggy PG warning

When pggrow_host thrasher kills a host and all its OSDs, those
OSDs stop sending lease ACKs. Then, once the lease interval (16s)
expires, PGs go into laggy state. This is expected for this
test scenario and thus adding it to the ignore list.

Signed-off-by: Ponnuvel Palaniyappan <pponnuvel@gmail.com>