ipahost: Honour update_password also for random

If random is enabled and update_password is limited to "create_only", the
random password may only be changed if the host does not exist yet.

Additionally the generation of the random password will fail, if the host
is already enrolled if update_password is "always" (default value). An
error will be reported early in this case now.

The command host_show is now used instead of host_find, as `has_password`
and `has_keytab` are only returned by host_show, but not by host_find. The
find_host function has been adapated for this change.

Resolves: #253 (ipahost is not idempotent)