Simplify allocDir.Build() function to avoid depending on client/structs,
and remove a parameter that's always set to `false`.
The motivation here is to avoid a dependency cycle between
drivers/cstructs and alloc_dir.
Fixes an issue where the Ruby runtime expects the sticky bit to be set
on the temp directory. The sticky bit is commonly set on the temp
directory since it is usually shared by many users. This change brings
ours in line with that assumption.
Fixes#2178 and allows using Docker and other image based drivers even
when nomad is run as a non-root user.
`client/allocdir` tests can be run as a non-root user to ensure this
behavior and tests that rely on root or non-root users properly detect
their effective user and skip instead of fail.
Mahmood Ali