Description of problem:

# ipa-server-install --dirsrv-config-file params.ldif
with params.ldif:
~~~
dn: cn=config
changetype: modify
replace: nsslapd-maxsasliosize
nsslapd-maxsasliosize: 50000000
replace: nsslapd-sasl-max-buffer-size
nsslapd-sasl-max-buffer-size: 50000000
~~~
ends up with nsslapd-sasl-max-buffer-size at 2MB anyway due to:
https://bugzilla.redhat.com/show_bug.cgi?id=1044193

So during install the new value is correctly taken into account, but reset to
2M at the end:
~~~
2017-12-13T16:53:18Z DEBUG only: set nsslapd-sasl-max-buffer-size to '2097152',
current value [u'50000000']
2017-12-13T16:53:18Z DEBUG only: updated value [u'2097152']
~~~

The default value of 2MB is proving too low in some environments for initial
replication, so overriding it safely is a must.


Version-Release number of selected component (if applicable):
As of RHEL 7.4


How reproducible:
Always


Steps to Reproduce:
1. Install IPA server with "--dirsrv-config-file params.ldif" as above
2. Watch the log

Actual results:
~~~
2017-12-13T16:53:18Z DEBUG only: set nsslapd-sasl-max-buffer-size to '2097152',
current value [u'50000000']
2017-12-13T16:53:18Z DEBUG only: updated value [u'2097152']
~~~


Expected results:
Custom value is not overridden by hardcoded default value.

Additional info:

We should detect if the value was changed.

Known workaround:
* set the parameter nsslapd-sasl-max-buffer-size to 50000000 on the first IdM
server right after installation
* install all replica with ipa-replica-install --dirsrv-config-file params.ldif
* once installation is done, set the nsslapd-sasl-max-buffer-size to 50000000
in the replica as well.

As initial replication is done after the new value (50MB) is used, but before
the hardcoded value of 2MB is set, the parameter's value goes from:

dirserv default => 50MB => 2MB => 50MB