Extra parameters are accepted for `exec` in the `prepare` context

[jktjkt]

Originally reported by @vxpyy9 in #27:

$ sysrepo-cli 
Connected to sysrepo [datastore target: operational]
/> prepare /ietf-netconf:get-config
(prepare: /ietf-netconf:get-config) > exec source/candidate
Couldn't send RPC: SR_ERR_VALIDATION_FAILED
 Mandatory choice "config-source" data do not exist. (path "/ietf-netconf:get-config/source") (SR_ERR_VALIDATION_FAILED)
 RPC input validation failed. (SR_ERR_VALIDATION_FAILED)