Support descriptor customization by allowing external setting variables in some fields

Currently the <envVarRef> element supports customization via system properties in 4 ways (see below). UIMA could also support the ${variable-name} syntax in these text fields. In addition it could also be supported in the value for the name and location attributes of the import element so that an aggregate's delegates could be controlled via external settings. This might also be done by changing the primitive descriptor as in 2 & 3 below, but in some cases a completely different delegate descriptor is preferable.

From the References document:
The syntax for environment variable references is <envVarRef>[VariableName]</envVarRef> , where [VariableName] is any valid Java system property name. Environment variable references are valid in the following places:
1) The value of a configuration parameter (String-valued parameters only)
2) The <annotatorImplementationName> element of a primitive AE descriptor
3) The <name> element within <analysisEngineMetaData>
4) Within a <fileResourceSpecifier> or <fileLanguageResourceSpecifier>