reuse STF_PROVIDER_HOST and provide an option to declare usbmuxd port only

[vdelendik]

mcloud-agent/roles/mac-devices/vars/main.yml.original

Line 27 in b8d0dbe

USBMUXD_SOCKET_ADDRESS: macos-agent.zebrunner.farm:2222

we have two places for the same host address: STF_PROVIDER_HOST and USBMUXD_SOCKET_ADDRESS
Let's reorganize a little bit renaming 2nd one into the USBMUXD_SOCKET_PORT

and in zebrunner-farm combine STF_PROVIDER_HOST:USBMUXD_SOCKET_PORT into the USBMUXD_SOCKET_ADDRESS env var for connector.

As a benefit - single place for defining provider host ip/name