Move sfc custom flavor creation

Move sfc custom flavor creation

The custom flavor that is needed for the sfc test case is created in the
tacker deployment phase.
The result of this is that when we use alternative methods of deploying
tacker (i.e. the tacker fuel plugin) the test fails.

Changes:
- Move the flavor creation inside the test case code
- Add an optional public parameter in the [get_or_]create_flavor utility
  functions.

Change-Id: Iea13f172076770dde3876b7e27edc9b7ca9b28db
Signed-off-by: George Paraskevopoulos <geopar@intracom-telecom.com>