Allow colon in dimension key

This Bugfix is based on https://bugs.launchpad.net/monasca/+bug/1668937

The monasca-api spec definition for dimensions allows the following (https://github.com/openstack/monasca-api/blob/master/docs/monasca-api-spec.md#metric-name-and-dimensions):

The dimension key and value strings may include any characters except the following: > < = { } ( ) , ' " \ ; &

There is no restriction on using COLON ':'.

__Expected behaviour:__

Querying for metrics with dimensions where the key and value contain a COLON is possible.

Example Query parameter:

{'start_time': '2017-02-28T06:20:51Z', 'dimensions': 'service:url:http://192.168.10.4:5601,service:name:monasca-api', 'name': 'http_status'}

In this situation the dimensions should be parsed as:

key: service:url, value: http://192.168.10.4:5601
key: service:name, value: monasca-api

__Observed behaviour:__

The dimension key-value pair is always splitted on the first occurred colon:

key: service, value: url:http://192.168.10.4:5601
key: service, name: monasca-api

To be able to understand what is the key and what is the value, we need to introduce a new syntax for defining dimensions.

For example we could surround them with quotes like:

{'start_time': '2017-02-28T06:20:51Z', 'dimensions': '"service:url":"http://192.168.10.4:5601","service:name":"monasca-api"', 'name': 'http_status'}

Quotes are already forbidden.

The change must be downward compatible. All old incoming request should be treated as it is now.