Prospector 1.4.1 fixes

[nishakm]

Major changes:

• Return None in mount_single_layer function when something goes
  wrong. Do a check for the target_dir string existing before passing
  it on for analysis.
• Return None for DriverManager try-except calls when there is a
  NoMatches exception. This modifies all instantiations of Stevedore's
  DriverManager class.
• Iterate over the dictionary using .items() rather than just the plain
  dictionary.
• Use "with" to open files and subprocess pipes. This may result in
  certain errors not being raised, but it is hard to know at this
  point.

Minor changes:

• Used the "maxsplit" argument in main.py as we only need the
  first item.
• Used [] rather than list() to initialize lists.
• Specify encoding as 'utf-8' when opening files.
• Update re-raises to explicitly state which exception they are
  re-raising from using "from".
• Use "enumerate" for looping through list's index while throwing
  out the unused value.
• Suppressed the "useless-suppression" message along with
  "too-many-branches" suppression due to pylint bug:
  useless-suppression false positive with ungrouped-imports pylint-dev/pylint#2366
• Wraped some lines that were too long, except in the HTML format
  file due to the need to keep the literal HTML formatting.
• Updated the year on some modified files.

Signed-off-by: Nisha K nishak@vmware.com