(MODULES-9578) Create authorized_key in root path

(MODULES-9578) Create authorized_key in root path

Previously, when the `target` property was set, the ssh_authorized_key
resource could not create directories/files within root-owned paths.
This behavior is due to the module switching context to the user, then
attempting to create the directory/file as the specified user,
ultimately failing because of insufficient permissions.

This commit adds a new parameter, `drop_privileges` which when set to
false allows the module to write a ssh_authorized_key file in a
privileged path. Due to the possible security implications of this,
the parameter must be manually specified in order to activate this
functionality.

A path is considered to be privileged/trusted if all of its ancestors:
- do not contain any symlinks
- have the same owner as the user who runs Puppet
- are not world/group writable