Improve siteFs

[rukkal]

This PR adds two improvements:

1. The function setupVolumeMapMounts now uses mkdir_p instead of the plain mkdir in order to create parent folders, if necessary. E.g. the following configuration in udiRoot.conf no longer requires the system administrator to use sitePreMountHook to create the parent folder:

siteFs=/var/opt/cray/alps:/var/opt/cray/alps:rec

2. Shifter could drop/skip data of the image when the system administrator configures siteFs to mount folders that clash with the contents of the image. We modified Shifter to emit a warning when potentially important data is being skipped.

[coveralls]

Coverage decreased (-0.2%) to 67.863% when pulling 5671540 on rukkal:improve_sitefs into 4dc2134 on NERSC:master.