get_bootstrap_from_recipes() result consistency

[AndreMiras]

In #1872 (comment) we realised get_bootstrap_from_recipes() was not always returning consistent result across setups.
This is an investigation task to determine if it's a bug or a feature and if it's a bug to fix it.

[AndreMiras]

Basically on my setup and in 0c9ef2e calling list(Bootstrap().list_bootstraps()) returns the following: ['webview', 'sdl2', 'service_only', 'empty'] I guess for @opacam and Travis it returned something with "service_only" being first.

@AndreMiras listdir depends on inode ordering and is not alphabetically sorted:

    @classmethod
    def list_bootstraps(cls):
        '''Find all the available bootstraps and return them.'''
        forbidden_dirs = ('__pycache__', 'common')
        bootstraps_dir = join(dirname(__file__), 'bootstraps')
        for name in listdir(bootstraps_dir):
            if name in forbidden_dirs:
                continue
            filen = join(bootstraps_dir, name)
            if isdir(filen):
                yield name

I think a simple sorted(listdir(bootstraps_dir)) may fix it and make it behave deterministically