(IAC-1186) Add new $use_servername_for_filenames parameter

Prior to this commit, the $filename variable, which is used to
construct the filename of Apache's various config and log files,
obtained it's default value from the $name parameter.

As #2064 highlights, it is possible for $name to contain spaces
in it and this can cause cumbersome log file names, albeit POSIX
compliant.

Also related is #2068, which changes the $filename variable source
from $name to $servername. This arguably seems more appropriate,
especially given that $servername defaults to $name if undefined.

This commit attempts to create a satisfactory solution to both #2064
and #2068 by introducing the $use_servername_for_filenames param.
When set to true, a sanitized $servername parameter value will be
used to construct $filename.

When undefined or set to $false, it will retain the existing
behaviour and use the $name parameter value.

This will default to false until the next major release (v6.0.0),
after which it will default to true. Then, in the subsequent major
release (v7.0.0) it will be deprecated altogether and the default
behaviour will be to use the sanitized value of $servername for the
$filename var.

Warning messages are output to the console to alert users of this
change in behaviour.