Description of problem:
It is possible to delete the instance 'slapd-standalone' with remove-ds.pl
tool, while specifying wrong prefix 'anyprefix-standalone'.

How reproducible:
Always

Steps to Reproduce:
1. Install an instance with a name 'standalone'
2. Run remove-ds.pl tool to remove it, but specify a wrong prefix instead of
'slapd'
$ remove-ds.pl -i 'anyprefix-standalone'

Actual results:
Instance anyprefix-standalone removed.

Expected results:
The following errors occurred during removal:
Error: could not find directory server configuration directory
'anyprefix-standalone'.  Error: No such file or directory
Error: could not remove directory server standalone

Additional info:
/usr/sbin/remove-ds.pl contains the next line (50):
my ($slapd, $inst) = split(/-/, $instname, 2);

So it doesn't check that before '-' is 'slapd' prefix and then it just splits
that part.