Add slow tag for test_ceilometer_resource_list

This commit adds a slow tag to the ceilometer cli test
test_ceilometer_resource_list. This test is consistently among the
slowest in a gate run. (usually slower than the scenario tests) So,
this shouldn't be part of a normal gate run a cli test is just a
simple sanity check it shouldn't take minutes to execute.

Change-Id: I8aa73823d5f6558c3e378ed8a0e17e351775b8dd
Merge "Add slow tag for test_ceilometer_resource_list"