res_rtp_asterisk: Add 'rtp show settings' cli command

This change introduce a CLI command for the RTP to display the general
configuration.

In the first step add the follow fields of the configurations:
  - rtpstart
  - rtpend
  - dtmftimeout
  - rtpchecksum
  - strictrtp
  - learning_min_sequential
  - icesupport

Change-Id: Ibe5450898e2c3e1ed68c10993aa1ac6bf09b821f