fix takeover cgi script

if last_backup was an int, the script failed with this error:

result undefined, global datetime = <module 'datetime' from '/srv/slapgrid/
slappart6...6d33cee74/lib/python2.7/lib-dynload/datetime.so'>,
datetime.datetime = <type 'datetime.datetime'>,
datetime.datetime.fromtimestamp = <built-in method fromtimestamp of type
object>, last_backup = '1587724103'
<type 'exceptions.TypeError'>: a float is required
      args = ('a float is required',)
      message = 'a float is required'