Description of problem:
If the user chooses to preserve the sssd configuration, IPA will overwrite the
existing configuration even with specifying --preserve-sssd

Version-Release number of selected component (if applicable):
ipa-client-3.0.0-25.el6.x86_64

How reproducible:
always

Steps to Reproduce:
1. create a sssd configuration in /etc/sssd/sssd.conf.
2. ipa-client-install --server=$MASTER --password=$ADMINPW --unattended
--realm=$RELM --domain=$DOMAIN --principal=$ADMINID --preserve-sssd
3. Observe that the contents of sssd.conf have been cleared.

Actual results:

The sssd config was wiped out and replaced.

Expected results:
I expect ipa to either error out, and not continue, or to integrate it's config
with the running config where possible.

Additional info:

For a valid, easy to set up sssd.conf, I suggest adding the following to
/etc/sssd/sssd.conf

[nss]
filter_users = root
filter_groups = root

    try:
        domain = sssdconfig.new_domain(cli_domain)
    except SSSDConfig.DomainAlreadyExistsError:
        root_logger.info("Domain %s is already configured in existing SSSD " +
            "config, creating a new one.", cli_domain)
        root_logger.info("The old /etc/sssd/sssd.conf is backed up and will " +
            "be restored during uninstall.")
        sssdconfig = SSSDConfig.SSSDConfig()
        sssdconfig.new_config()
        domain = sssdconfig.new_domain(cli_domain)