Add ignored repositories list

[m-gorecki]

This allows to specify a list of repositories names that will be ignored by upgrade command. The list should be placed inside project.yml file and look for example like this:

project.repositories.ignored: - apache-mynewt-nimble - stm-cmsis_device_f3 - stm-stm32f3xx_hal_driver

apache-mynewt-core repository can't be ignored.