Allow to specify a credentials file when mounting

Most of our users already have a credentials file for Samba shares (used by pyNeighborhood and automated scripts). In older versions of pyNeighborhood we just added something like "credentials=/home/mvaldez/.samba_creds.txt" to the mount options, but now we can't as pyNeighborhood appends its own "credentials" or "guest,sec=none" options. Please allow to enter a credentials file in the UI or don't try to add automatically "guest,sec=none" if "credentials" is already set for the mount options.