Check if flavors exist before creating them.

* lib/tempest: When creating the m1.tiny and m1.nano flavors ensure that
they don't exist first. This is important for Grenade where code may be
run multiple times and should expect that some things preexist.

Change-Id: I1772d4334f39d612f8a187eb5311a1b2caee3953
Merge "Check if flavors exist before creating them."
Merge branch 'master' of github.com:openstack-dev/devstack