s4-dsdb-test: Implement samdb_connect_env() to rely solely on environment

s4-dsdb-test: Implement samdb_connect_env() to rely solely on environment

this is to help me port Python tests to be more Unit test alike
and remove all global handling
Starting from a new test suite - tombstone_reanimation.py

Andrew Bartlett rose his concerns that passing parameters
through environment may make tests hard to trace for
failures. However, passing parameters on command line
is not Unit test alike either. After discussing this with him
offline, we agreed to continue this approach, but prefix
environment variables with "TEST_". So that an env var
should not be used by coincidence.

Change-Id: I29445c42cdcafede3897c8dd1f1529222a74afc9
Signed-off-by: Kamen Mazdrashki <kamenim@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>