ACCUMULO-1944 Fix coverage for functional tests

The -C flag for test/system/auto/run.py did not work. This changeset
allows it to work again if a functional Cobertura installation is
placed under $ACCUMULO_HOME/lib/test/cobertura.

The code for producing instrumented Accumulo code was inactive and
out of date. It was reworked so that, if -C is passed, the Accumulo
JARs are instrumented and placed into a location ahead of their
standard location in the test classpath. (If -C is not passed, any
instrumented JARs are removed.) The classpath is also dynamically
adjusted to include whatever Cobertura JAR is available; its name
includes a version number (as of 2.0.x).

The command-line scripts shipped with Cobertura 2.0.x are out of
date and do not work out of the box. Pull request #102 was submitted
to cobertura/cobertura on Github to fix the problem; in lieu of that,
the scripts must be manually updated to use a correct classpath.

Tested with Cobertura 2.0.3 / 2.0.4-SNAPSHOT with needed script
updates.
ACCUMULO-1944 Restore test runs when Cobertura not installed

The changes in the prior commit to re-enable test coverage assume that Cobertura
is available. When it isn't, functional test runs fail. This commit allows runs
to proceed when Cobertura is not installed.