20550 - allow activemq ssl certificate and key to be set via environment variables

This commit adds to ability to set the ActiveMQ SSL cert and private key
as environment variables. The environment variables will be checked
before values in the config file.

The private key is defined as MCOLLECTIVE_ACTIVEMQ_POOLx_SSL_KEY, where
x is the pool number.

The cert is defined as MCOLLECTIVE_ACTIVEMQ_POOLx_SSL_CERT, where x is
the pool number.