Ignore '+' sign at the beginning of member lists.

It seems to be valid to prefix member lists with a '+' sign which is ignored by the perl config parser. check-mk does this all the time:

```define host {
  host_name                     example
  use                           check_mk_host
  address                       a.b.c.d
  _TAGS                         foo bar
  _FILENAME                     /example.mk
  hostgroups                    +Example-Routers
  contact_groups                +Example-Contacts
}
  define hostgroup {
   hostgroup_name                Example-Routers
   alias                         Example-Routers
 }```

The perl parser does **not** add the host to the corresponding hostgroups/contactgroups due the prefixed '+' sign, but nagios is doing so. This breaks other tools parsing the configuration and rely on group memberships (i.e. smokegios).

The small patch just drops any prefix '+' sign on ARRAY like attributes. I did not find any documentation about the semantic of the '+' sign. So this patch is not realy a fix, but a workaround to get atleast some of the group assignments working.


HTH,
Thomas