Add shared functions to module_utils/powershell.ps1 and refactor powershell modules to utilize the common powershell code

Add shell_plugins to abstract shell-specific functions out of runner, add winrm connection plugin, add initial Windows modules.

Fixes password lookup docs

Allow gem executable to contain spaces

Allow npm executable to contain spaces

rbenv has a special exec function which loads the correct ruby version before executing a command.
The syntax for this is `/usr/local/rbenv/bin/rbenv exec gem ...` for example.
But previously when specifying executable='/usr/local/rbenv/bin/rbenv exec gem' this would not work because the string was treated as one executable. This PR fixes that by splitting the executable on spaces.

NVM has a special script which loads the correct node version before executing a command.
The syntax for this is `/usr/local/nvm/nvm-exec ...`, so `nvm-exec npm list --json` for example.
But previously when specifying `executable='/usr/local/nvm/nvm-exec nvm'` this would not work because the string was treated as one executable.

Update tower.rst