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
Merge "Move negative tests for v3 test_server_metadata"
Merge "Move common _delete_volume cleanup method in base compute test class"