Expand `config[:synced_folders]` paths before genarating Vagrantfile

Relative source paths were written as is, leading to paths relative to
the Vagrantfile. This expands them to relative to the kitchen root.

Also move `%{instance_name}` and option expanding out from the
Vagrantfile template so that custom templates don't need to copy the
logic.

Fixes: #70
Improve `synced_folders` documentation

Modify the example to use relative path and `%{instance_name}`.
Improve documentation for setting the Vagrant provider

Add note and link to Vagrant docs for `VAGRANT_DEFAULT_PROVIDER`
environment variable which can be set to specify the default provider.
Support vagrant-softlayer plugin syntax
Merge pull request #72 from tmatilai/relative_synced_folders

Fix relative synced folder paths
Merge pull request #76 from tmatilai/default_provider

Improve documentation for setting the Vagrant provider
Remove default memory setting

The memory setting overrides the default specified both by the box and
in global `~/.vagrant.d/Vagrantfile`. Most Vagrant base boxes (including
Bento boxes) default to 386 MB, which should be fine for normal
Test Kitchen usage. And removing the default allows users to set their
own host-wide default.

Ref: #22
Merge pull request #80 from zambon/feature-support-vagrant-softlayer

Support vagrant-softlayer plugin syntax
Use more YAML in synced folders
Merge pull request #82 from test-kitchen/sethvargo/update_readme

Use more YAML in synced folders
Update README.md

driver_usage URL needs some redirection - This is perhaps a close approximation for now.
Merge pull request #88 from tjsoftworks/patch-1

Update README.md
Release v0.15.0
Remove comma from example