Only use nproc to determine functional test concurrency when it's available in the os.

Some operating systems don't ship which nproc, which causes impala-config.sh to fail. This
change alleviates the problem by checking if nproc exists, and setting a reasonable
default if it fails.

Change-Id: Ic6e4d0fbce57eedc82163cfa17f71bdccbc38b51