Support for cpu_quota and cpu_period

[shin-]

Fixes #766

[aanand]

Minor thing, but this style of conditional makes me do a double-take - plus we're immediately doing another check in the second branch. I think a single outer check and two inner checks would be more normal.

Also, could we use version_lt? I find it much easier to read.

if cpu_quota:
    if not isinstance(cpu_quota, int):
        raise TypeError(...)
    if version_lt(version, '1.19'):
        raise errors.InvalidVersion(...)
    host_config['CpuQuota'] = cpu_quota

Same for the cpu_period conditional.

[shin-]

Should be good now.

[aanand]

LGTM