Enforce function declaration format in bash8

Check that function calls look like ^function foo {$ in bash8
See commit: https://github.com/openstack-dev/devstack/commit/aee18c749b0e3a1a3a6907a33db76ae83b8d41d9

Change-Id: Ia5aab4c428e93ea431eb8cec6c20aaade07354e1