mon: LogMonitor: appropriately expand channel meta variables

mon: LogMonitor: appropriately expand channel meta variables

We must only expand the log file's channel meta variables upon requiring
a channel's log file.  As we may have a 'default' channel that will
cover all channels, we must wait to expand channels as they come in and
do so if they haven't yet been expanded.  Expanding the 'log_file' in
place would have the unfortunate side effect of expanding, say,

default=/tmp/whatever.$channel.log

to

default=/tmp/whatever.default.log

which would not be what we wanted upon receiving a message that should
go into channel 'foo' -- assuming we specified no such channel in the
options, channel 'foo' should go into '/tmp/whatever.foo.log'.

Signed-off-by: Joao Eduardo Luis <joao@redhat.com>