Make setpgid configurable for exec commands

[amCap1712]

#1494 added the setpgid setting to ensure signals are propagated to all processes and not just the subshell. However, this might not always be desirable.

For instance, servers like uWSGI run workers in subprocesses and wait for workers to finish processing active requests for a while before restarting them to support graceful reloading. consul-template sending the reload signal to all processes, sends singals to these worker processes in uWSGI as well which can interfere with graceful reloads.

Currently, we are working around this by using the array form of the command which does not set the pgid on the child process. But I feel it might be useful to make this option configurable as well.

If the flag is unset, the existing behavior is retained.

[hashicorp-cla]

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