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 this value being taken from existing my.cnf.
'mount_point' paramenter is removed from all trove-*.conf
files.

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

Fixes bug 1248212

Change-Id: I2631f7b7cd0de9a13a20bd986b7f3fe8d363bf0e