Add Flags class to base.Context

Add Flags class to base.Context

This patch adds a Flags class to represent the flags passed to
the context.

Flags will contain a set of attributes (for example no_setup and no_teardown).
Those attributes will modify the behaviour of the context.

The attributes added initially are:
* 'no_setup' (default: False)
* 'no_teardown' (default: False)

In future patches, they will be used by the context to:
* Change the name of the context.
* Skip the deploy() and undeploy() methods to keep the VNFs active after the
  testcases are run.

JIRA: YARDSTICK-886
Change-Id: I734b082aa101649eea8991304a8b753ae96aa9de
Signed-off-by: Rodolfo Alonso Hernandez <rodolfo.alonso.hernandez@intel.com>
Signed-off-by: Emma Foley <emma.l.foley@intel.com>