engine: Remove UserSessionTimeOutInvalidationInterval

Currently, there are two different config options
for controlling Engine user session expiration:

 1. UserSessionTimeOutInterval:
    - delay between Engine startup and first
      execution of "cleanExpiredUsersSessions" job
    - allowed values -1,1..100000 (-1 = disable)
    - default value is 30 min

 2. UserSessionTimeOutInvalidationInterval:
    - delay between subsequent executions of
      "cleanExpiredUsersSessions" job
    - allowed values -1,1..100000 (-1 = disable)
    - default value is 30 min

This patch removes the second config option, using
first option to represent both delay intervals.

While it makes little sense to use -1 to disable
Engine user session expiration entirely (as this
essentially generates memory leaks), this patch
preserves the original "-1,1..100000" range for
backward compatibility.

Change-Id: Ic6eb6273653bfffe0d1fa1b8a4dc39e085c19496
Signed-off-by: Vojtech Szocs <vszocs@redhat.com>