adds options for client certs to downloader class

Adds :client_cert to the env array and then passes it into the curl
options array
adds param for client certs

adds an option to the command parser for client certicates
adds client cert
adds client cert to default config
adds accessor for client_cert
passes client cert option from Vagrant file to box_add
Merge branch 'master' into client-certs
implemetn array arguments from #1569
Updating for NFS Fix
Cleanup for Pull Request
clarification: don't -> shouldn't
Call the rest of the middleware stack all the time.
add ansible.host_key_checking configuration parameter
need_configure should return true when master_config is defined as well.

This fixes #2204 and fixes saltstack/salty-vagrant#100
core: Report the proper invalid state when waiting for the guest machine to be ready
Merge pull request #2205 from jcockhren/fix_2204

provisioners/salt: need_configure should return true when master_config is defined as well.
Merge pull request #2203 from copperlight/ansible-host-key-checking

provisioners/ansible: add ansible.host_key_checking configuration parameter
Update CHANGELOG
Merge pull request #2200 from mikerowehl/virtualbox_destroy_unused_interfaces_fix

providers/virtualbox: call rest of destroy middleware stack
Merge branch 'master' of https://github.com/mitchellh/vagrant