tests samba_tool: fix flapping user-virtualCryptSHA test

tests samba_tool: fix flapping user-virtualCryptSHA test

Fix flapping test, occasionally a password would be generated that failed
the password criteria, which resulted in the test user not being
created.  The tests relying on this user being present then failed.

This patch ensures that the generated password contains at least one digit,
at least one upper case letter and at least one lower case letter.
The generated passwords do not contain special characters to avoid shell
escaping issues.

Signed-off-by: Gary Lockyer <gary@catalyst.net.nz>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>
Reviewed-by: Andreas Schneider <asn@samba.org>
Autobuild-User(master): Garming Sam <garming@samba.org>
Autobuild-Date(master): Mon Aug  7 05:34:24 CEST 2017 on sn-devel-144