Add --runtimedir config parameter for specifying relative location of all directories/files

In case you want to run multiple MaxScale instances on one computer, it is necessary to explicitly specify numerous directories and files.

To make it easier to do that, there should be one flag using which (almost all) directorylocations can be specified. E.g.

maxscale --runtimedir=/maxscale/instance1

That the would case MaxScale to look for everything else relative to that directory

/maxscale/instance1/etc/maxscale.cnf

/maxscale/instance1/var/log/maxscale/maxscale.log

/maxscale/instance1/var/cache/maxscale/

...

This flag would be almost the same as -- basedir, the difference being that --basedir also specifies the location of the binary files.

In practice,-- basedir is mostly useful for developers and other who may want to run multiple maxscale versions concurrently or one after the other, while -- runtimedir would be useful for MaxScale end-users who want to run different MaxScale instances (of the same version) concurrently.