Make context types constants

Make context types constants

Make context types constants. These are the current context types:
  - Dummy
  - Heat
  - Kubernetes
  - Node
  - StandaloneOvsDpdk
  - StandaloneSriov

Any reference to a context type should use the referred constant.

JIRA: YARDSTICK-1292

Change-Id: I1ff0085161f88304d71e3ddad7975f9020e0fc20
Signed-off-by: Rodolfo Alonso Hernandez <rodolfo.alonso.hernandez@intel.com>