Fix a typo in ipa-adtrust-install help

"Add SIDs for existing users andgroups as the final step" changed
to "Add SIDs for existing users and groups as the final step".
Fix migration for openldap DS

openldap server does not store its schema in cn=schema entry, but
rather in cn=subschema. Add a fallback to ldap2 plugin to read from
this entry when cn=schema is not found. ldap2 plugin uses the schema
when doing some of the automatic encoding, like an automatic
encoding of DN object.

IPA migration plugin DN attribute processing is now also more
tolerant when it finds that some DN attribute was not autoencoded.
It tries to convert it to DN on its own and report a warning and
continue with user processing when the conversion fails instead of
crashing with AssertionError and thus abandoning the whole
migration run.

https://fedorahosted.org/freeipa/ticket/3372
Remove unused krbV imports

https://fedorahosted.org/freeipa/ticket/3381
Use fully qualified CCACHE names

Some parts of install scripts used only ccache name as returned by
krbV.CCache.name attribute. However, when this name is used again
to initialize krbV.CCache object or when it is used in KRB5CCNAME
environmental variable, it fails for new DIR type of CCACHE.

We should always use both CCACHE type and name when referring to
them to avoid these crashes. ldap2 backend was also updated to
accept directly krbV.CCache object which contains everything we need
to authenticate with ccache.

https://fedorahosted.org/freeipa/ticket/3381