ovs-bugtool: Ability to collect number of logs through plugins.

ovs-bugtool: Ability to collect number of logs through plugins.

A new option "--log-days" recently added lets us to control the
number of rotated logs included in the debug bundle. This option
only works on log files defined inside the ovs-bugtool code.

This patch lets us to do the same with logs collected through plugins.

The example format inside a plugin is:
<files type="logs">
    /var/log/one
</files>

This will collect one, one.[1-x], one.[1-x].gz. Where 'x' is 20
by default and can be controlled by the option '--log-days' passed
to ovs-bugtool.

Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>