Small server action code compression

Make the server action type calls use the same common function,
instead of a code duplication.
It is just the beginning of making the tempest.service more compact.

This patch also removes the duplicated not used methods from the json
client:
* confirm_migration (confirmResize)
* start_server (os-start)
* stop_server (os-stop)

Change-Id: Ic036354c93b837d455f9478dc6dac249ec674ae7
Merge "Small server action code compression"