rgw: add default_idle_timeout to allow override

Globally overriding the rgw idle_timeout is not possible because it it
needs to be done on a per client.0, client.1, etc. basis. Add the
default_idle_timeout key to the rgw config : it defaults to the
previously hardcoded default (30) and can be changed via the override.

The existing tasks that were previously overriding the idle_timeout on a
per client basis are changed to use the default_idle_timeout instead for
consistency and to allow a global override.

Signed-off-by: Loic Dachary <loic@dachary.org>