handle case where multiple uaa service instances are bound

[rlindner81]

our own afc-api app has two uaa services bound, one for communicating with our own services and one for the external facing api

currently mtx tool implicitly selects the first binding and runs with it.

it would be better to offer some configuration to override this selection either by number or even by name.

the general configuration format of .mtxrc is probably too simplistic to even hold this type of configuration, so this would need to be extended.

ideally the configuration should be changeable ad-hoc via MTX_UAA_APP or permanently.

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the easiest way to cover all this would be not to change the configuration format away from a string, but rather encode more information into the string.

for example

• afc-api:1 could mean use the second service binding or
• afc-api:afc-api-uaa could mean use the service binding with the name afc-api-uaa

pro

• support current approach for permanent config and ad-hoc env var config
• ok easy to implement

con

• logically breaks the naming because app now describes the app as well as the service binding
• since it really describes the binding and not the app, it's also incomplete, it should really be afc-api:xsuaa:1 to identify the service as well