should support plugin installation with env var options

[hieptranquoc]

make sure vagrant-libvirt plugin installed correctly.

[hoatle]

plugin installation requires some more options, so we're going to support that kind of options so that we can install the plugin via yaml config

https://github.com/vagrant-libvirt/vagrant-libvirt#installation

If you have issues building ruby-libvirt, try the following:

CONFIGURE_ARGS='with-ldflags=-L/opt/vagrant/embedded/lib with-libvirt-include=/usr/include/libvirt with-libvirt-lib=/usr/lib' GEM_HOME=~/.vagrant.d/gems GEM_PATH=$GEM_HOME:/opt/vagrant/embedded/gems PATH=/opt/vagrant/embedded/bin:$PATH vagrant plugin install vagrant-libvirt

[hoatle]

required by teracyhq-incubator/teracy-dev-essential#10

[hoatle]

note: I'm not sure if we can support to install the plugin out of the box.

Otherwise, this could work with our normal plugin config (if this work, we should follow this for simpler plugin config schema):

$ CONFIGURE_ARGS='with-ldflags=-L/opt/vagrant/embedded/lib with-libvirt-include=/usr/include/libvirt with-libvirt-lib=/usr/lib' GEM_HOME=~/.vagrant.d/gems GEM_PATH=$GEM_HOME:/opt/vagrant/embedded/gems PATH=/opt/vagrant/embedded/bin:$PATH vagrant up