Increase solver timeout or allow timeout to be configured dynamically

When working with berkshelf 3 under a large environment, the default timeout of 1s does not provide enough time to resolve dependency conflicts.

    $ berks install
    Resolving cookbook dependencies...
    Fetching 'foo' from source at cookbooks/foo
    Fetching cookbook index from http://localhost:26200...
    There is a dependency conflict, but the solver could not determine the precise cause in the time allotted.

For testing purposes, increasing the hardcoded limit in `lib/solve/solver.rb` from `1_000ms` to `10_000ms` results in the conflict being found.

    $ berks install
    Resolving cookbook dependencies...
    Fetching 'foo' from source at cookbooks/foo
    Fetching cookbook index from http://localhost:26200...
    Unable to satisfy constraints on package bar due to solution constraint (fake ~> 1.0).