Use bash-style test for Fedora version

Old-style test fails on Ubuntu when python3 enabled, with:

.../devstack/inc/python: line 52: [: 16.04: integer expression expected

Use bash-style test, which doesn't attempt to evaluate the RHS if the
LHS evaluates to false

Change-Id: If18031ab98c9060e5825c3a8d3c647bd3705cd9c
Closes-Bug: #1796174

inc/python

@@ -49,7 +49,7 @@ function get_python_exec_prefix {
     fi
     $xtrace
 
-    if python3_enabled && [ "$os_VENDOR" = "Fedora" -a $os_RELEASE -gt 26 ]; then
+    if python3_enabled && [[ "$os_VENDOR" == "Fedora" && $os_RELEASE -gt 26 ]]; then
         # Default Python 3 install prefix changed to /usr/local in Fedora 27:
         # https://fedoraproject.org/wiki/Changes/Making_sudo_pip_safe
         echo "/usr/local/bin"