Tools: Add key for MariaDB repository

Packages from the MariaDB repository were not automatically installed
by Puppet because the signing key was missing in Tools.

The key was identified as 0xcbcb082a1bb943db per
https://mariadb.com/kb/en/installing-mariadb-deb-files/ and downloaded
by:

| gpg --no-default-keyring                         \
|     --keyring modules/toollabs/files/mariadb.gpg \
|     --keyserver keyserver.ubuntu.com             \
|     --recv-key 0xCBCB082A1BB943DB

The content of the keyring can be verified by:

| gpg --no-default-keyring                         \
|     --keyring modules/toollabs/files/mariadb.gpg \
|     --fingerprint

Change-Id: I59a32281da2b66779765f662b4a10e0a1594876e