provider: explicitly stringify output of mysql query

[LadyNamedLaura]

Otherwise the result might corrupt the transactionstore.

For some reason some fields of the instance were still of the type Puppet::Util::Execution::ProcessOutput when puppet tries to serialize these into the transactionstore.yaml it adds the type.

When on a next run puppet tries to load the transactionstore.yam it thinks it's corrupted.

Transaction store file /opt/puppetlabs/puppet/cache/state/transactionstore.yaml is corrupt ((/opt/puppetlabs/puppet/cache/state/transactionstore.yaml): Tried to load unspecified class: Puppet::Util::Execution::ProcessOutput); replacing
Wrapped exception:
Tried to load unspecified class: Puppet::Util::Execution::ProcessOutput

[mcrauwel]

lgtm, tests are green!

[mcrauwel]

Thanks Simon!