skip refreshing assets cache while no database is configured

fixes cc before setup finished
Merge remote-tracking branch 'vivid/3.6' into 3.6
also pass config files writeable check if folder is writable
silently ignore apc_store returning false

this silently ignores the "Failed saving metadata to metadataCache" exception when apc is used.
Merge branch '3.6'
Change solr default path if no subdomains defined

If no subdomains are defined now root is used as value for %subdomain%.
So it's not needed to set solr path explicitely in every web.