allow switchboard to choose which incoming call to answer
do nothing when a talking switchboard answers another
rename unique_id variables
display caller id on the phone when switchboard retrieves a call

Problem was: the phone displayed "Anonymous" <Anonymous>
rename switchboard_unhold to switchboard_retrieve

retrieve is now used for unholding and for answering
improve logging when switchboard retrieves call
fix inversed cid_num and cid_name in AMIClass
fix test on XivoAMI.switchboard_retrieve
Merge branch '4218-switchboard-choose-incoming-call'

Conflicts:
	xivo-ctid/xivo_cti/ctiserver.py
	xivo-ctid/xivo_cti/services/current_call/manager.py
	xivo-ctid/xivo_cti/services/current_call/tests/test_current_call_manager.py
	xivo-ctid/xivo_cti/tests/test_xivo_ami.py
	xivo-ctid/xivo_cti/xivo_ami.py