samba-tool user: hashlib.sha1 is always present

samba-tool user: hashlib.sha1 is always present

We maybe thought we were checking that sha1 was in hashlib, but we were
only checking that hashlib is in the Python library (`hashlib.sha1()`
would not raise ImportError).

The documentation says hashlib always contains sha1 -- if that
changes, it is better we know by failing noisily with the import error
at the top of the file.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15756

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andreas Schneider <asn@samba.org>