slurmctld: ignore secondary group members that have no valid login

This avoids a warning message which is repeated on each reconfiguration of slurm
and which is due to a dangling group configuration in LDAP entries.

The error occurs when traversing the secondary group members of a given group
name, when trying to add these to a configured group. If these secondary group
members have no valid login (e.g. disabled via LDAP configuration), the error
is repeated on each reconfigure of slurm.

The error is harmless: since the users have no valid login, they can not log into
the system anyway. I have raised the issue described below with our LDAP admin,
there was no reply (likely since not important enough).

Since slurm is not a tool to debug the work of system administrators, and since
the secondary group members can not log in anyway, this patch replaces the error
message with a comment; it leaves untouched the positive case of found secondary
group members that have successfully been added to a configured group due to 
having a valid passwd/LDAP login entry.

Here is the case which gets repeated on our system, showing that each error message
corresponds to a 'no such user' error when trying to look up the user id:

-----------------------------------------------------------------------------------------------
[2011-03-29T08:19:35] error: Could not find user baradmin in configured group csstaff
[2011-03-29T08:19:35] error: Could not find user mvalle in configured group csstaff
[2011-03-29T08:19:35] error: Could not find user puradm in configured group csstaff
[2011-03-29T08:19:35] error: Could not find user ggobbi in configured group csappli
[2011-03-29T08:19:35] error: Could not find user mvalle in configured group csappli
-----------------------------------------------------------------------------------------------

palu2:0 ~>getent group csstaff
csstaff:*:1000:baradmin,biddisco,jfavre,mvalle,puradm
palu2:0 ~>id baradmin
id: baradmin: No such user
palu2:1 ~>id mvalle
id: mvalle: No such user
palu2:1 ~>id puradm
id: puradm: No such user

==> The secondary group members 'biddisco' and 'jfavre' are ok, no warnings.  
-----------------------------------------------------------------------------------------------
palu2:1 ~>getent group csappli
csappli:*:1010:ajocksch,alam,amangili,annaloro,biddisco,cordery,cponti,fgilles,ggobbi,grenker,jfavre,mgg,mvalle,nstring,piccinal,robinson,soumagne,tack,tadrian,uvaretto,wsawyer
palu2:0 ~>id ggobbi
id: ggobbi: No such user