introduce pip_install_gr

This creates a new pip_install_gr that installs from global
requirements allowed versions. Now that stable branches are getting
capped all of devstack needs to be fixed to do things like this.

Change-Id: I8fd0ef2bfc544ca2576fab09d3018f760b8848fe

lib/databases/mysql

@@ -155,7 +155,7 @@ EOF
 
 function install_database_python_mysql {
     # Install Python client module
-    pip_install MySQL-python
+    pip_install_gr MySQL-python
     ADDITIONAL_VENV_PACKAGES+=",MySQL-python"
 }
 

lib/databases/postgresql

@@ -104,7 +104,7 @@ EOF
 
 function install_database_python_postgresql {
     # Install Python client module
-    pip_install psycopg2
+    pip_install_gr psycopg2
     ADDITIONAL_VENV_PACKAGES+=",psycopg2"
 }