Use the full path to `chef-solo` and `chef-client`

This fixes a bug where a cookbook (like rbenv, rvm, chruby) sets the default system Ruby. Chruby and RBenv specifically use $PATH munging, making those binaries unavailable on subsequent Test Kitchen runs.

Since other people can "touch" `/usr/local`, assuming the symlinks in `/usr/local/bin` will point to the correct places isn't a valid assumption. I bit myself with this locally when cooking up some magic. I think specifying the full path is the right thing to do.

/cc @schisamo @yzl
@sethvargo So @portertech is correct on this. `/opt/chef/bin` is not a symlink to `/opt/chef/embedded/bin` the `gem` bin only exists under `/opt/chef/embedded/bin`:

```
vagrant@master-ubuntu-1204:~$ ls -la /opt/chef/
total 20
drwxr-xr-x 4 root root 4096 Jul 29 12:55 .
drwxr-xr-x 5 root root 4096 Jul 29 13:00 ..
drwxr-xr-x 2 root root 4096 Jul 29 12:55 bin
drwxr-xr-x 7 root root 4096 Jul 29 12:55 embedded
-rw-r--r-- 1 root root 1747 Apr 30 00:31 version-manifest.txt
vagrant@master-ubuntu-1204:~$ ls -la /opt/chef/bin/
total 72
drwxr-xr-x 2 root root 4096 Jul 29 12:55 .
drwxr-xr-x 4 root root 4096 Jul 29 12:55 ..
-rwxr-xr-x 1 root root  466 Apr 30 00:30 chef-apply
-rwxr-xr-x 1 root root  467 Apr 30 00:30 chef-client
-rwxr-xr-x 1 root root  476 Apr 30 00:30 chef-service-manager
-rwxr-xr-x 1 root root  466 Apr 30 00:30 chef-shell
-rwxr-xr-x 1 root root  465 Apr 30 00:30 chef-solo
-rwxr-xr-x 1 root root  480 Apr 30 00:30 chef-zero
-rwxr-xr-x 1 root root  472 Apr 30 00:30 coderay
-rwxr-xr-x 1 root root  468 Apr 30 00:30 erubis
-rw-r--r-- 1 root root    0 Apr 30 00:26 .gitkeep
-rwxr-xr-x 1 root root  479 Apr 30 00:30 htmldiff
-rwxr-xr-x 1 root root  461 Apr 30 00:30 knife
-rwxr-xr-x 1 root root  476 Apr 30 00:30 ldiff
-rwxr-xr-x 1 root root  460 Apr 30 00:30 ohai
-rwxr-xr-x 1 root root  456 Apr 30 00:30 pry
-rwxr-xr-x 1 root root  462 Apr 30 00:30 rackup
-rwxr-xr-x 1 root root  487 Apr 30 00:30 restclient
-rwxr-xr-x 1 root root  460 Apr 30 00:30 shef
vagrant@master-ubuntu-1204:~$ ls -la /opt/chef/embedded/
total 28
drwxr-xr-x 7 root root 4096 Jul 29 12:55 .
drwxr-xr-x 4 root root 4096 Jul 29 12:55 ..
drwxr-xr-x 2 root root 4096 Jul 29 12:55 bin
drwxr-xr-x 6 root root 4096 Jul 29 12:55 include
drwxr-xr-x 5 root root 4096 Jul 29 12:55 lib
drwxr-xr-x 8 root root 4096 Jul 29 12:55 share
drwxr-xr-x 4 root root 4096 Jul 29 12:55 ssl
vagrant@master-ubuntu-1204:~$ ls -la /opt/chef/embedded/bin/
total 1636
drwxr-xr-x 2 root root   4096 Jul 29 12:55 .
drwxr-xr-x 7 root root   4096 Jul 29 12:55 ..
-rwxr-xr-x 1 root root    472 Apr 30 00:31 autospec
-rwxr-xr-x 1 root root    471 Apr 30 00:26 bundle
-rwxr-xr-x 1 root root    472 Apr 30 00:26 bundler
lrwxrwxrwx 1 root root      3 Apr 30 00:26 captoinfo -> tic
-rwxr-xr-x 1 root root   8573 Apr 30 00:26 clear
-rwxr-xr-x 1 root root   4273 Apr 30 00:26 c_rehash
-rwxr-xr-x 1 root root   4407 Apr 30 00:26 erb
-rwxr-xr-x 1 root root    897 Apr 30 00:26 gem
-rw-r--r-- 1 root root      0 Apr 30 00:26 .gitkeep
-rwxr-xr-x 1 root root  72961 Apr 30 00:26 infocmp
lrwxrwxrwx 1 root root      3 Apr 30 00:26 infotocap -> tic
-rwxr-xr-x 1 root root    328 Apr 30 00:26 irb
-rwxr-xr-x 1 root root  46587 Apr 30 00:26 makedepend
-rwxr-xr-x 1 root root   5326 Apr 30 00:26 ncurses5-config
-rwxr-xr-x 1 root root   5330 Apr 30 00:26 ncursesw5-config
-rwxr-xr-x 1 root root 594438 Apr 30 00:26 openssl
-rwxr-xr-x 1 root root 593744 Apr 30 00:26 pkg-config
-rwxr-xr-x 1 root root    450 Apr 30 00:30 rake
-rwxr-xr-x 1 root root    798 Apr 30 00:26 rdoc
lrwxrwxrwx 1 root root      4 Apr 30 00:26 reset -> tset
-rwxr-xr-x 1 root root    198 Apr 30 00:26 ri
-rwxr-xr-x 1 root root    469 Apr 30 00:31 rspec
-rwxr-xr-x 1 root root  11959 Apr 30 00:26 ruby
-rwxr-xr-x 1 root root  18277 Apr 30 00:26 tabs
-rwxr-xr-x 1 root root  77602 Apr 30 00:26 testgdbm
-rwxr-xr-x 1 root root    308 Apr 30 00:26 testrb
-rwxr-xr-x 1 root root  77897 Apr 30 00:26 tic
-rwxr-xr-x 1 root root  18880 Apr 30 00:26 toe
-rwxr-xr-x 1 root root  18842 Apr 30 00:26 tput
-rwxr-xr-x 1 root root  29028 Apr 30 00:26 tset
```