Bug 1816784

This bug is created as a clone of upstream ticket:
https://pagure.io/freeipa/issue/8240

### Issue
ipa-kra-install fails on a cluster with KRA already installed if all the KRA-enabled replica are hidden.

#### Steps to Reproduce
1. install master (DNS, CA)
2. install replica0 (DNS, CA) as hidden
3. install KRA on replica0 (so: on the hidden replica)
4. install KRA on master

#### Actual behavior
Lookup failed: Preferred host master.ipa.test does not provide KRA.
Failed to find an active KRA server!

#### Expected behavior
KRA successfully installed.

#### Version/Release/Distribution
Any up to at least 2020-3-24 IPA from git master.

#### Additional info:
- this is because ipa-kra-install does:
```
                config.kra_host_name = find_providing_server(
                    'KRA', api.Backend.ldap2, [api.env.ca_host]
                )
```
and masters.find_providing_servers()  does:
```
def find_providing_servers(svcname, conn=None, preferred_hosts=(), api=api):
(...)
        if ENABLED_SERVICE in cfgstrings:
            servers.append(servername)
        # use hidden services on preferred hosts
        elif HIDDEN_SERVICE in cfgstrings and servername in preferred_hosts:
            servers.append(servername)
```
e.g. in the absence  of preferred_hosts, find_providing_servers() returns an empty list.

The workaround is therefore to set ca_host in default.conf to the hidden replica's FQDN right before ipa-kra-install, and remove that setting right after ipa-kra-install.