libcli/auth: use unique key_name values in netlogon_creds_cli_context_common()

libcli/auth: use unique key_name values in netlogon_creds_cli_context_common()

Until all callers are fixed to pass the same 'server_computer'
value, we try to calculate a server_netbios_name and use this
as unique identifier for a specific domain controller.

Otherwise winbind would use 'hostname.example.com'
while 'net rpc testjoin' would use 'HOSTNAME',
which leads to 2 records in netlogon_creds_cli.tdb
for the same domain controller.

Once all callers are fixed we can think about reverting this
commit.

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>