Fix salt-cloud ec2 requests containing spaces.

salt-cloud receives an error when trying to create an EC2 instance when
any number of argument values contain a space. This happens because
salt-cloud is incorrectly building the url string when calculating the
signature. AWS signature algorithm #2 requires that spaces be encoded
using %20, however python's urlencode method replaces them with a +.
The easy fix is to just search and replace in the encoded string.

I encountered this myself with a security group with a space in it.
This was reported in issue #10181
https://github.com/saltstack/salt/issues/10181

The error looks like this:
someinstancename:
    ----------
    Errors:
        ----------
        Error:
            ----------
            Code:
                SignatureDoesNotMatch
            Message:
                The request signature we calculated does not match the signature you provided. Check your AWS Secret Access Key and signing method. Consult the service documentation for details.
    RequestID:
        12345678-9abc-def0-1234-56789abcdef0