Use create_test_server for attach volume server test

This commut uses create_test_server to clean up the
test test_create_server_attach_volume, which currently
suffers from a higher-than-usual failure rate. Example:

This commit also replaces manual volume creation with
``create_volume`` helper function defined in Tempest's
base compute class.

http://logs.openstack.org/47/496047/1/check/gate-tempest-dsvm-patrole-admin-ubuntu-xenial/2901c0f/console.html#_2017-08-22_01_40_22_359577

Change-Id: If85279ff2f4ee7fad2b3d4c665398f699afc7b38
1 file changed