compute: add get_serial_console rpc and cells api calls

Co-Authored-By: Ian Wells <iawells@cisco.com>
Co-Authored-By: Sushma Korati <sushma_korati@persistent.co.in>
Co-Authored-By: Sahid Orentino Ferdjaoui <sahid.ferdjaoui@redhat.com>

Partial-Implements: blueprint serial-ports
Change-Id: If79255be351e76fb4c4f671c929b71ce822a9db2
Merge "compute: add get_serial_console rpc and cells api calls"