DRY and other fixes in pip module
DRY and other fixes in pip module

This makes the following adjustments:

- DRY fix for repeated search paths
- Checks that python bin found in virtualenv is executable
- Removes string-escape encoding, it doesn't have any use if we're not
  dealing with quotes/special characters
- Removes redundant islink check, since a symlink will return True when
  it points to a file and not a directory, and we know thanks to the
  prior if block that the path is not a directory so it can only be a
  file.
- `pip` is almost never referred to in upper case, log messages changed
  to reflect this.
- Make context caching of pip version specific to bin_env

saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt