Load flags before creating logger so the logs path considers the
standalone flag.

Move the log file path definition to the function, otherwise it will
calculated during import and (most likely) before defining the
flags.STANDALONE value.

Create logger inside `run_frontend` right after knowing if we are
standalone or not.

- Resolves: #7512

NullHandler doesn't have anymore the argument bubble.
https://pythonhosted.org/Logbook/changelog.html#version-0-11-0

we need the 'multi' feature all the same, so we'll be depending on
logbook >= 0.7.0.

there's an error in the import

for the version in ubuntu trusty, the call for the zeromq handler
initialization gets one less argument

Fix:
If the logbook controller is not started the stop call will fail.
Trying to start it twice fails.

Prevent locks caused by the zmq log handler reaching the zmq's HWM (High
water mark / buffer limit) and causing some components to block until
the buffer is empty (running the zmq handler for instance).
We run the zmq handler in the background all the time to prevent this.

- Resolves: #7222

Due to #7139 (the logbook log centralizer) logs from 3rd party libs are
included in the centralized logs with lots of debug information and we
don't want that.
We use the existing silencer to exclude logs that are not from leap
modules.

- Resolves: #7185

The logger is the first thing to be created and on a first run the
config path won't exist. This way we make sure the path always exists.