Adding SCP option to file_write, file_ensure and file_upload

To get around problems with file_write failing on large files, using
SCP : have added scp=False as an optional parameter to file_write, file_ensure, file_upload

limitations:
  requires host and user to have been initialised
  will fail silently (should add a file_ensure check after?)

An alternative approach would be to use scpclient (with modifications) that implements scp on top of paramiko  - this has its own issues and would require a modified package to be installed - so possibly too complex?

Feel free to comment and will update.