The main idea of the commit is to fix code
according with the latest oslo.i18n requirements
https://docs.openstack.org/oslo.i18n/latest/
1. removed log translation if log is not seen by users
in raised exception or api call response.
2. keep translated log if it's used in raised exception.
3. removed log message 'Error while reading %s'
which was "dead" (unused) code in the function
"_get_value_from_conf_file"
of module "agent/linux/dhcp.py".
Partial-Bug: 1600788
Change-Id: Ifb5455336b06c2c87a930b816c90b4a766856b1e
c156c7d883