Swift pollster silently return no counter if keystone endpoint is not present

Fixes bug 1131359

Change-Id: I861d9e75fb6e77eabe7e8e519875676faf7ddf3a
Updated the description of get_counters routine.

Modified the second counter description of
get_counters.

Fixes: bug 1156814

Change-Id: Ie7d4d3d93f853598069d8676f6be4335ddd3afb9
Merge "Swift pollster silently return no counter if keystone endpoint is not present"
Merge "Updated the description of get_counters routine."