Goal: Quay-side failover between multiple LDAP endpoints

Background: Currently it appears the Quay only supports a single LDAP server or DNS entry. It would be helpful if multiple LDAP servers could be added to allow for failover. LDAP HA  is typically achieved by automatic client failover based on the processing of DNS SRV records (ldap://_ldap._tcp.doimain.com...) but python-ldap doesn't support this.

Requirements:

• a new config switch called LDAP_URIS which can hold multiple LDAP URIs seperated by whitespace
• if present, the LDAP_URIS parameter is preferred and looped through when attempting to connect to LDAP until a working backend has been found.
• no other settings (base dn, passwords, admin dn, etc) would differ between the multiple LDAP endpoints, so no endpoint-specific settings will be supported