Add enforce support to schedule pipeline config

[amimas]

Currently the scheduled pipeline configuration syntax is like this:

   group-name/project-name:
    schedules:
      Title of the schedule:
        ref: <branch-name>
        cron: <cron-expression>
        cron_timezone: <time-zone>
        variables:
          PIPELINE_VARIABLE1:
            value: <variable-value>

Consider the following 2 cases:

• Rename the "Title of the schedule" to something else
• Delete the entire schedule

In the first case, a new schedule will be created; existing schedule will remain in the project. For the 2nd case, nothing happens. The schedule from previous run of gitlabform will remain in the project.

In most cases the above is probably not expected. It'd be good to have enforce: true support for the schedules config so that any "unconfigured schedules" are deleted.

[gdubicki]

Released in v3.7.0 a moment ago. Thanks again for your contribution, @amimas! 😊