Changes

hlr: wrap f_vty_subscr_create and _delete

Add wrappers as f_subscr_create/_delete (without _vty), and only use the
VTY implementation for OsmoHLR. This is in preparation for implementing
a different method for PyHSS in a follow-up patch.

Avoid passing the VTY parameter by splitting each function into a normal
variant that runs on test_CT, and a _HCH variant that runs on
HLR_ConnHdlr.

Change-Id: I6aafa118645346fd709c56de1e38771ed4de93b7