s3-gse: move krb5 fallback to smb_gss_krb5_import_cred wrapper

s3-gse: move krb5 fallback to smb_gss_krb5_import_cred wrapper

MIT krb5 1.9 version of gss_krb5_import_cred() may fail when importing
credentials from a keytab without specifying actual principal.
This was fixed in MIT krb5 1.9.2 (see commit
71c3be093db577aa52f6b9a9a3a9f442ca0d8f20 in MIT krb5-1.9 branch, git
master's version is bd18687a705a8a6cdcb7c140764d1a7c6a3381b5).

Move fallback code to the smb_gss_krb5_import_cred wrapper. We only
expect this fallback to happen with krb5 GSSAPI mechanism, thus hard
code use of krb5 mech when calling to gss_acquire_cred.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=12611

Signed-off-by: Alexander Bokovoy <ab@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>
Autobuild-User(master): Alexander Bokovoy <ab@samba.org>
Autobuild-Date(master): Wed Mar  8 22:00:24 CET 2017 on sn-devel-144