CLI - dsconf inst config should support multi-valued operations

[droideck]

Issue Description
We have two editable multi-valued attributes in cn=config: nsslapd-haproxy-trusted-ip and nsslapd-referral.

Our current cn=config implementation doesn't support bunch ADD operations.
And while it'll be too much work to refactor our cn=config (and I'm not sure we need that at all), we still should
make our CLI tools more robust so it can handle multi-valued attributes correctly.

[droideck]

There are two more issues that needs to be fixed/enhanced:

• After a restart, some values of the configuration parameter nsslapd-haproxy-trusted-ip are gone - add operation is not handled correctly.

• It would be useful if the "dsconf config replace" command can be used to replace current value(s) with new ones.
  For instance, to replace the existing values of "nsslapd-haproxy-trusted-ip":

  dsconf -D "cn=Directory Manager" ldap://<HOST>:<PORT> config replace nsslapd-haproxy-trusted-ip=<NEW_IP_ADDRESS_1> nsslapd-haproxy-trusted-ip=<NEW_IP_ADDRESS_2> nsslapd-haproxy-trusted-ip=<NEW_IP_ADDRESS_3>