Make non-public Cloudera connector dependencies optional

[turu]

When using liquibase on Cloudera cluster via a binary release, it's convenient to have a liquibase-impala fat-jar with all necessary (non-public) Hive&Impala dependencies bundled inside. However by setting strict compile dependency on those libraries, we are preventing ourselves from being able to:

1. Release liquibase-impala to a public Maven repository, so that people running liquibase via Maven plugin can start using liquibase-impala with no extra-installation steps at all (provided that they already have Cloudera connectors or are planning to use other drivers instead: see Add support for Apache Hive(2) Driver #5 ).
2. Make a binary release on Github, so that people running liquibase via a binary can start using liquibase-impala with no installation steps at all (provided that they already have Cloudera connectors or are planning to use other drivers instead: see Add support for Apache Hive(2) Driver #5 ) - other than downloading a jar from Github that is.
3. Allow people to use liquibase-impala with different versions of Hive/Impala drivers than those declared in liquibase-impala pom.

To alleviate this situation, we could make non-public (or all non-essential) Cloudera dependencies optional and place them in a Maven profile (see https://cwiki.apache.org/confluence/display/MAVENOLD/Profiles+for+optional+dependencies). That way, all of the below scenarios would be easily achievable:

1. Creating a fat-jar with all present Cloudera dependencies bundled inside - just like now.
2. Releasing liquibase-impala to a public Maven repo, without hardcoded dependencies on any particular drivers, so that people using liquibase via Maven plugin could use any driver in any version they like.
3. Creating a binary release on Github, without any driver dependencies bundled inside, so that people using a binary release of Liquibase can use any driver in any version they like.