slapproxy: Add local software release path

Include a local software release root path concept in slapproxy to ease adapting the local SR urls when restoring a backup'd slapproxy in a different computer, e.g. for resilience.

The local_software_release_root parameter is a path that can be passed via slapos.cfg:

[slapproxy]
local_software_release_root = <path>

or via the command line using:

slapos proxy start --local-software-release-root <path>

When slapproxy starts, it updates all local URLs that are subpaths of the previous local software release root path to be subpaths of the new one:

<previous-root-path>/<subpath>  --->  <new-root-path>/<subpath>

The default root path is /. This means that by default slapproxy behaves the same as before.