constants: Move most paths to separate module

This is inpreparation for the implementation of virtual clusters. Many
paths will change based on an environment variable and are no longer
constant and should no longer be in “constants.py”. Since “constants.py”
is already huge a number of other paths are also moved in the process.

For now a wildcard import is used to re-export all paths from
“constants.py” (this will change over coming patches). Daemon log paths
have been changed to use a function. “RUN_GANETI_DIR” was moved to
“RUN_DIR” as the latter was only used in “constants.py”.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>