fix installation failure if pip is not installed yet

With "-e" flag, "$?" syntax does not work. If the line before "$?" exits
other than 0, the scripts stops at the line and never reach the next
line.

Closes-Bug: #1327918
Change-Id: I9de2392d972c8dbdb0df3ea4ce2e256fd9cb723c

install_rally.sh

@@ -110,8 +110,7 @@ install_system_requirements() {
     unsupported_os_system_requirements
   fi
 
-  hash pip 2> /dev/null
-  if [ $? -ne 0 ]; then
+  if ! hash pip 2> /dev/null; then
     curl -L -o ${GETPIPPY_FILE} ${PIP_SECURE_LOCATION}
     python ${GETPIPPY_FILE}
   fi