Customize Vagrantfile with environment variables

This patch adds the ability for some `PDAL_VAGRANT_` prefixed
environment variables to customize the behavior of PDAL's Vagrantfile.
The general policy is to provide a 'switch' environment variable that
enables or disables a given behavior when the environment variable is
set to *any* value. For example, the variable
`PDAL_VAGRANT_DISABLE_NFS`, predictably, disables nfs mounting. A few
special cases actually use the value of the environment variable for
customization, usually some sort of integer value (e.g.
`PDAL_VAGRANT_VIRTUALBOX_MEMORY`).

This patch adds the following environment variables:

- `PDAL_VAGRANT_PORT_80_FORWARD`: set or disable forwarding from guest
  port 80. If an integer value, set the host port. If non-integer or
  zero, disable port forwarding for guest port 80.
- `PDAL_VAGRANT_VIRTUALBOX_MEMORY`: set or disable a custom memory value
  for a VirtualBox vm.
- `PDAL_VAGRANT_VIRTUALBOX_CPUS`: set or disable a custom cpu value for
  a VirtualBox vm.
- `PDAL_VAGRANT_VIRTUALBOX_IOAPIC`: set VirtualBox ioapic value.
- `PDAL_VAGRANT_VIRTUALBOX_ENABLE_GUI`: set to any value to turn on the
  VirtualBox gui.
- `PDAL_VAGRANT_PRIVATE_NETWORK_IP`: set or diable a private network IP
  for the vm.
- `PDAL_VAGRANT_DIABLE_NFS`: disable nfs mounting

The following values were added previously in
35b355f39b5d88801eeb80dfe038df05ed6c72fb, and are included here for
completeness:

- `PDAL_VAGRANT_SSH_FORWARD_AGENT`: enable ssh agent forwarding.

This patch closes #311.
Merge pull request #313 from PDAL/issues/311-configure-vagrant-with-envvars

Customize Vagrantfile with environment variables