Add tests for cynara_admin_list_policies_descriptions

Add tests for cynara_admin_list_policies_descriptions

Each of added tests uses the fact that every cynara test is wrapped in
CynaraTestEnv that saves cynara service plugins.
Every test clears cynara service plugins folder and restarts cynara.
Then proper combination of plugins (sometimes none) are loaded.
There are 4 tests added:
* tc30_admin_list_descriptions_no_plugins - no plugins are loaded,
 there are only default cynara policies;
* tc31_admin_list_descriptions_1_plugin_single_policy;
* tc32_admin_list_descriptions_1_plugin_multiple_policy;
* tc33_admin_list_descriptions_multiple_plugins.

Change-Id: Id778477c35a6d65a7d4f71df03dcccefcbb6e756