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/metadata.yaml b/installers/charm/pol/metadata.yaml
index f9839b9..e01293b 100644
--- a/installers/charm/pol/metadata.yaml
+++ b/installers/charm/pol/metadata.yaml
@@ -43,3 +43,6 @@
     interface: kafka
   mongodb:
     interface: mongodb
+  mysql:
+    interface: mysql
+    limit: 1