Ask for admin password in ipa-adtrust-install
The credentials of the admin user will be used to obtain Kerberos ticket before
configuring cross-realm trusts support and afterwards, to ensure that the
ticket contains MS-PAC information required to actually add a trust with Active
Directory domain via 'ipa trust-add --type=ad' command.
https://fedorahosted.org/freeipa/ticket/2852