[macos-10.12] Allow Packer to pick up memory user var. (#768)
[macos-10.12] Allow Packer to pick up memory user var. (#768)

For some reason with Pack 0.12.3, the current `"memory"` user variable
is not being picked up and a default value of `1024` is always being
used, leading to macOS 10.12 not being able to finish its installation.
For some reason, changing the user variable from `"memory"` to almost
anything else (in this case, `"memsize"`) allows the user variable to be
picked up.

Note that this was observed with the `vmware-iso` builder and may be the
only affected one. I verified this behavior by running:

```
env PACKER_LOG=1 bundle exec bento build --only=vmware-iso --debug macos-10.12
```

Signed-off-by: Fletcher Nichol <77a0fd9e8048bbacd11af4e957bc6ff03b549f49@nichol.ca>
juliandunn/bento,chef/bento,vincentaubert/bento,juliandunn/bento,norcams/bento,b1-systems/bento,vincentaubert/bento,Oefenweb/bento,norcams/bento,norcams/bento,devopxteam/bento,chef/bento,techmaniack/bento,devopxteam/bento,techmaniack/bento,devopxteam/bento,ConSol/bento,juliandunn/bento,chef/bento,Oefenweb/bento,ConSol/bento,techmaniack/bento,ConSol/bento,b1-systems/bento,devopxteam/bento,vincentaubert/bento