Document expected marketplace service icon, and rescale preserving original H/W ratio

As a service broker author, in order to properly configure by service catalog with the imageUrl serving an appropriate image, I need to discover expected image format (height/width ratio, min/max size and transparent background support)

As a stratos users, in order to enjoy a nice looking markerplace page despite untuned service broker catalogs, I expect marketplace service icons to be displayed resized while preserving original Height/weight ratio

Expected behaviour

• A doc page in the stratos repo discoverable using the imageUrl keyword, which provides hints to service broker authors/operator.
• A better image rescale style to preserve H/W ratio.

Actual behaviour

Service broker author/operator without front-end skills to guess expected format could provide image and an invalid ratio found on the internet such as served from http://cassandra.apache.org/img/cassandra_logo.png which would display as :

Other users would use browser dev tools to inspect the css and infer displayed size (48 pixel x 48 pixel ?)

.service-icon[_ngcontent-c57] img[_ngcontent-c57], .service-icon[_ngcontent-c57] mat-icon[_ngcontent-c57] {
    font-size: 48px;
    height: 48px;
    width: 48px;
}

Steps to reproduce the behavior

A service broker which returns http://cassandra.apache.org/img/cassandra_logo.png as the imageUrl field.

Start by searching the repository for imageUrl and the marketplace service icon styles shown in the issue. Review how the icon is rendered and where service-broker guidance is documented. Done means the repository has discoverable imageUrl guidance and marketplace icons preserve their original height-to-width ratio.