Switch to using neutron.common.utils:replace_file()

Switch to using neutron.common.utils:replace_file()

neutron.agent.linux.utils:replace_file() and
neutron.common.utils:replace_file() have same functionality.

This is the 1st patch in the series of 4 patches.
It modifies neutron.common.utils:replace_file(),
so it can be used by all components as a replacement
for neutron.agent.linux.utils:replace_file().
New keyword parameter 'file_mode=0o644' is added
to neutron.common.utils:replace_file().

Partial-bug: #1504477
Change-Id: Id1a7f1236786e8606c91bb9925cd9ac8e95892b3