Add environment variables to CircleCI config generation script

The purpose of this ticket is adding arguments to the CircleCI config generation script allowing to set the values specific environment variables such as DTEST_REPO or DTEST_BRANCH in the generated .circleci/config.yml file. For example, we could generate a CircleCI config file with MIDRES specifying a dtest repo and branch by running:

generate.sh -m \
  -e DTEST_REPO=git://github.com/adelapena/cassandra-dtest.git \
  -e DTEST_BRANCH=CASSANDRA-8272 

Or we could set the test multiplexer for repeating a specific test with HIGHRES:

generate.sh -h \
  -e REPEATED_UTEST_TARGET=testsome \
  -e REPEATED_UTEST_CLASS=org.apache.cassandra.cql3.ViewTest \
  -e REPEATED_UTEST_METHODS=testCompoundPartitionKey,testStaticTable \
  -e REPEATED_UTEST_COUNT=100

This can be useful on its own so we don't have to manually edit the config-2_1.yml/config.yml, and it can also be useful for automation scripts manipulating these files.