Fix logic for config file env variables

This commit fixes the logic around the 2 env variables to set the
location of the tempest config file. Previously the check to see
whether the config variables should be used instead of the default
path would be too selective and the default path etc/tempest.conf
would be used. The only check needed is whether the specified file
exists or not. The presence of the env variables shouldn't affect
that logic.

Change-Id: Id6be0f067673d600bf04dea700e69c2baffb9d9f
Move common _delete_volume cleanup method in base compute test class

The test_volumes_get and test_volumes_list v2 compute tests were doing
similar volume cleanup.  This patch moves that code into the base class
so other tests that need to do volume cleanup, i.e. test_server_rescue,
can use it.

Related-Bug: #1254772

Change-Id: I59f4d638f8ab6e95736c78a384607ca89f1dcb67
Move negative tests for v3 test_server_metadata

Move negative tests from test_server_metadata.py to its negative file

Partially implements blueprint negative-test-files

Change-Id: I02a05828e26be357728ff73adf75b469fce1b594
Updated from global requirements

Change-Id: I54f89ffa49d3fdc8cc71528faef7b10f115370b2
Merge "Move negative tests for v3 test_server_metadata"
Merge "Move common _delete_volume cleanup method in base compute test class"
Merge "Fix logic for config file env variables"
Merge "Updated from global requirements"