(PDK-739) Fall back to default template if necessary

[rodjek]

Checks if the repo pointed to by the saved template-url from the answer file contains valid refs and fall back if it doesn't. git ls-remote will return non-zero (or specifically 2) if the remote does not exist or does not contain the specified ref (if supplied).

[coveralls]

Coverage increased (+0.02%) to 93.219% when pulling ace5f8a on rodjek:pdk-739 into 1633768 on puppetlabs:master.

[coveralls]

Coverage increased (+0.02%) to 93.222% when pulling 10a5785 on rodjek:pdk-739 into 1633768 on puppetlabs:master.

[bmjen]

@rodjek I've pushed an update to prevent the warning from getting spammed during a module creation b/c 'default_template_url' is called a few times.

[~/proj/sdk/test_modules]$ pdk new module foo
pdk (WARN): Unable to access the previously used template 'http://github.com/mjen/pdk-templates', using the default template instead.
pdk (WARN): Unable to access the previously used template 'http://github.com/mjen/pdk-templates', using the default template instead.
pdk (WARN): Unable to access the previously used template 'http://github.com/mjen/pdk-templates', using the default template instead.
pdk (WARN): Unable to access the previously used template 'http://github.com/mjen/pdk-templates', using the default template instead.
pdk (WARN): Unable to access the previously used template 'http://github.com/mjen/pdk-templates', using the default template instead.
pdk (WARN): Unable to access the previously used template 'http://github.com/mjen/pdk-templates', using the default template instead.
pdk (INFO): Creating new module: foo

I've modified it to update the answers file to remove the template-url the first time we detect an invalid url.

This is the new output.

[~/proj/sdk/test_modules]$ pdk new module foo
pdk (WARN): Unable to access the previously used template 'http://github.com/mjen/pdk-templates', using the default template instead.
pdk (INFO): Creating new module: foo
``

[coveralls]

Coverage decreased (-0.07%) to 93.129% when pulling 3a041f4 on rodjek:pdk-739 into 1633768 on puppetlabs:master.

[coveralls]

Coverage increased (+0.02%) to 93.222% when pulling 64a10ca on rodjek:pdk-739 into 1633768 on puppetlabs:master.