Allow customize http vhost config using HttpdCustomization.CustomConfigSecret

[stuggi]

This change allows to customize the httpd vhost config using this parameter to specify a secret that contains service config data. The content of each provided snippet gets rendered as a go template and placed into /etc/httpd/conf/httpd_custom__ .
At the end of the vhost config in the default httpd template these custom configs get included using Include conf/httpd_custom_<endpoint>_*.

For information on how sections in httpd configuration get merged, check section "How the sections are merged" in https://httpd.apache.org/docs/current/sections.html#merging

All possible parameters which can be use in a template can be looked up in the -config-data secret of the service like:

$ oc get secret -n openstack barbican-config-data -o json | jq -r .data.TemplateParameters | base64 -d 
$ oc get secret -n openstack barbican-api-config-data -o json | jq -r .data.TemplateParameters | base64 -d

or in the running pod of the service in the file:
$ cat /var/lib/config-data/default/TemplateParameters

The content is a versioned dump of the parameters of the service operator, like:

DatabaseConnection: mysql+pymysql://user:pwd@openstack.<namespace>.svc/keystone?read_default_file=/etc/my.cnf EnableSecureRBAC: true
EnabledSecretStores: simple_crypto
GlobalDefaultSecretStore: simple_crypto
KeystoneAuthURL: http://keystone-internal.openstack.svc:5000 LogFile: /var/log/barbican/barbican-api.log
ServicePassword: <pwd>
ServiceUser: barbican
SimpleCryptoEnabled: true
SimpleCryptoKEK: ""
TimeOut: 90
TransportURL: rabbit://rabbitmq-secret/fake
VHosts:
  internal:
    Override: true
    ServerName: barbican-internal.<namespace>.svc
    TLS: false
  public:
    Override: true
    ServerName: barbican-public.<namespace>.svc
    TLS: false

Depends-On: openstack-k8s-operators/lib-common#591
Depends-On: openstack-k8s-operators/lib-common#593

Jira: https://issues.redhat.com/browse/OSPRH-13100

[openshift-ci]

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[Deydra71]

nit: Is actually TemplateParameters, not templatingParameters as described in commit message, if someone'd want to check, otherwise lgtm