Moves negative tests from test_server_metadata

This change set moves the negative tests in
tempest/api/compute/servers/test_server_metadata to
a new file test_server_metadata_negative

Also removes usage of modules directly and qualifies
test_server_create_metadata_key_too_long as negative test.
 ** seprates test into individuals
Note: HACKING.rst
The second requirement is that all negative tests must be added to a negative
test file. If such a file doesn't exist for the particular resource being
tested a new test file should be added

Change-Id: I91f1ee898866e458ff15a4d5035a772ff5c14701
Add simple node group tmpl API test for Savanna

It tests CRUD and list operations for node group templates.

Change-Id: Ib2c94db50db03998ad402d52b4f87032581fdc76
Merge "Moves negative tests from test_server_metadata"
Merge "Add simple node group tmpl API test for Savanna"