Resolve external members from trusted domain via Global Catalog
A sequence is following:
1. Match external member against existing trusted domain
2. Find trusted domain's domain controller and preferred GC hosts
3. Fetch trusted domain account auth info
4. Set up ccache in /var/run/ipa_memcached/krb5cc_TD<domain> with principal ourdomain$@trusted.domain
5. Do LDAP SASL interactive bind using the ccache
6. Search for the member's SID
7. Decode SID
8. Replace external member name by SID