Remove "nova-manage flavor list" CLI test

The "nova-manage flavor" sub-commands were deprecated in Juno and Nova
change I1798c8a0697009b3dcebeb818580c8323880e395 removes them.

This removes the test_flavor_list test and changes the test_debug_flag
and test_verbose_flag tests to use 'nova-manage service list' instead of
'nova-manage flavor list'.

Change-Id: I4e2399f2fbab2c20299dd62c104b9220c1d36fb2
Merge "Remove "nova-manage flavor list" CLI test"