Test for npm availability in sudo context

npm proxy provider runs
@machine.communicate.sudo(command).

cap provider verifies availability of npm with `which' NOT
using sudo.

This may result in:
- which succeeds for the ssh user (mostly vagrant)
- machine.communicate.sudo('npm do something') does not
  succeed because sudo target user does not have the same
  PATH variable configured.

This may result in the following error message breaking the
Vagrant environment completely:

The following SSH command responded with a non-zero exit status.
Vagrant assumes that this means the command failed!

npm config set proxy http://some.configured.proxy.here:8080/

Stdout from the command:

Stderr from the command:

bash: line 2: npm: command not found