outposts/ldap: Rework/improve LDAP search logic. (#1687)

* outposts/ldap: Refactor searching so we key primarily off base dn

* docs: Updating guides on sssd and the ldap outpost.

internal/outpost/ldap/server/base.go

@@ -19,6 +19,7 @@ type LDAPServerInstance interface {
 
 	GetBaseDN() string
 	GetBaseGroupDN() string
+	GetBaseVirtualGroupDN() string
 	GetBaseUserDN() string
 
 	GetUserDN(string) string
@@ -32,4 +33,7 @@ type LDAPServerInstance interface {
 
 	GetFlags(string) (flags.UserFlags, bool)
 	SetFlags(string, flags.UserFlags)
+
+	GetBaseEntry() *ldap.Entry
+	GetNeededObjects(int, string, string) (bool, bool)
 }