(SDK-331) allow additional gems to be installed

[DavidS]

• bundle install --path ignores all "system" installed gems and causes unnecessary package installs
• bundle install (without --path) installs into GEM_HOME, which by default is non-user writeable

To still use the pre-installed packages, but allow folks to install additional gems
this sets GEM_HOME to the user's cachedir, and put all other cache locations onto GEM_PATH

See https://stackoverflow.com/a/11277228 for background

This still requires that the exact required gem versions are available in
package_cachedir.

[DavidS]

I've verified this fix on my local machine by installing a hand-built gem into the pdk native package:

david@davids:~/tmp/testmod$ pdk validate --debug
pdk (INFO): Running all available validators...
pdk (DEBUG): Using 'private/ruby/2.1.9/bin/bundle' from PDK package
[|] Checking for missing Gemfile dependenciespdk (DEBUG): Executing '/opt/puppetlabs/pdk/private/ruby/2.1.9/bin/bundle check --gemfile=/home/david/tmp/testmod/Gemfile'
[✖] Checking for missing Gemfile dependencies
The following gems are missing
 * puppet (5.0.1)
 * mcollective-client (2.11.1)
Install missing gems with `bundle install`

pdk (DEBUG): Using 'private/ruby/2.1.9/bin/bundle' from PDK package
pdk (DEBUG): Executing '/opt/puppetlabs/pdk/private/ruby/2.1.9/bin/bundle install --gemfile=/home/david/tmp/testmod/Gemfile'
[✔] Installing missing Gemfile dependencies
pdk (DEBUG): Executing '/home/david/tmp/testmod/bin/metadata-json-lint --format json /home/david/tmp/testmod/metadata.json'
[✔] Checking metadata (metadata.json)
pdk (DEBUG): Executing '/home/david/tmp/testmod/bin/rubocop --format json /home/david/tmp/testmod'
[✔] Checking Ruby code style
david@davids:~/tmp/testmod$