start hook fails because of impossible innodb_buffer_pool_size in my.cnf

[Saverio Proto]

Please note that this bug impacts the user experience in the GettingStarted guide:
https://jujucharms.com/docs/stable/getting-started

Steps to reproduce:

juju deploy cs:bundle/mediawiki-single

You will have then a Hook failure in the mysql charm:

ubuntu@xenial:~$ juju status --format=yaml
model:
  name: bug-model
  controller: lxd-test
  cloud: localhost
  region: localhost
  version: 2.0.3
machines:
  "0":
    juju-status:
      current: started
      since: 19 Feb 2017 11:58:55+01:00
      version: 2.0.3
    dns-name: 10.16.172.173
    ip-addresses:
    - 10.16.172.173
    instance-id: juju-4f2fc8-0
    machine-status:
      current: running
      message: Running
      since: 19 Feb 2017 11:58:29+01:00
    series: trusty
    hardware: arch=amd64 cores=0 mem=0M
  "1":
    juju-status:
      current: started
      since: 19 Feb 2017 11:59:02+01:00
      version: 2.0.3
    dns-name: 10.16.172.192
    ip-addresses:
    - 10.16.172.192
    instance-id: juju-4f2fc8-1
    machine-status:
      current: running
      message: Running
      since: 19 Feb 2017 11:58:38+01:00
    series: trusty
    hardware: arch=amd64 cores=0 mem=0M
applications:
  mediawiki:
    charm: cs:trusty/mediawiki-3
    series: trusty
    os: ubuntu
    charm-origin: jujucharms
    charm-name: mediawiki
    charm-rev: 3
    exposed: false
    application-status:
      current: unknown
      since: 19 Feb 2017 12:01:22+01:00
    relations:
      db:
      - mysql
    units:
      mediawiki/0:
        workload-status:
          current: unknown
          since: 19 Feb 2017 12:01:22+01:00
        juju-status:
          current: idle
          since: 19 Feb 2017 12:01:23+01:00
          version: 2.0.3
        leader: true
        machine: "0"
        public-address: 10.16.172.173
  mysql:
    charm: cs:trusty/mysql-29
    series: trusty
    os: ubuntu
    charm-origin: jujucharms
    charm-name: mysql
    charm-rev: 29
    exposed: false
    application-status:
      current: error
      message: 'hook failed: "start"'
      since: 19 Feb 2017 12:01:20+01:00
    relations:
      cluster:
      - mysql
      db:
      - mediawiki
    units:
      mysql/0:
        workload-status:
          current: error
          message: 'hook failed: "start"'
          since: 19 Feb 2017 12:01:20+01:00
        juju-status:
          current: idle
          since: 19 Feb 2017 12:01:20+01:00
          version: 2.0.3
        leader: true
        machine: "1"
        public-address: 10.16.172.192

Debug the start Hook:
juju debug-hooks mysql/0 start

the start Hook just calls:
service mysql restart

The problem is that in my.cnf there is this impossible value:
innodb_buffer_pool_size = 13455327232

The mysql error log complains it is not possible to allocate so much memory

170219 11:02:27 InnoDB: Initializing buffer pool, size = 12.5G
InnoDB: mmap(13770686464 bytes) failed; errno 12
170219 11:02:27 InnoDB: Completed initialization of buffer pool
170219 11:02:27 InnoDB: Fatal error: cannot allocate memory for the buffer pool
170219 11:02:27 [ERROR] Plugin 'InnoDB' init function returned error.
170219 11:02:27 [ERROR] Plugin 'InnoDB' registration as a STORAGE ENGINE failed.
170219 11:02:27 [ERROR] Unknown/unsupported storage engine: InnoDB
170219 11:02:27 [ERROR] Aborting