Add integration between pol and mysql charms

- Added mysql_uri config in POL charm
- Added mysql relation endpoint in POL charm

Change-Id: I7983c1b83da6d894af864669b918215611848bf7
Signed-off-by: David Garcia <david.garcia@canonical.com>
diff --git a/installers/charm/pol/config.yaml b/installers/charm/pol/config.yaml
index 59f695b..909235e 100644
--- a/installers/charm/pol/config.yaml
+++ b/installers/charm/pol/config.yaml
@@ -27,3 +27,8 @@
   mongodb_uri:
     type: string
     description: MongoDB URI (external database)
+  mysql_uri:
+    type: string
+    description: |
+      Mysql URI with the following format:
+        mysql://<user>:<password>@<mysql_host>:<mysql_port>/<database>