Remove 'mount_point' from guestagent RPC API

'mount_point' is used inconsistently across guestagent code.
Sometimes its value is taken from parameters of RPC call
and sometimes from CONF. This patch makes CONF the only
source of the value.

Blueprint remove-mount-point-from-parameters-for-instance-prepare

Fixes bug 1248212

Change-Id: I2631f7b7cd0de9a13a20bd986b7f3fe8d363bf0e