By default, don't depend on mysql in ewok-env

[climbfuji]

Is your feature request related to a problem? Please describe.
At the moment, mysql is a hard dependency in ewok-env, and because it's tricky to install and comes with narrow version constraints on boost, we've either installed it manually and made it available as an external package, or we've come up with workarounds to build a canned/internal boost just for mysql. The reality, mysql is only needed in one special case, which is
running JEDI-Skylab in localhost mode. This only happens on macOS laptops at JCSDA, on the JEDI Singlenode AMI for AWS, and on AWS ParallelCluster. HPCs, non-JCSDA user installations, and other cloud systems can be installed w/o mysql.

Describe the solution you'd like
As discussed in JCSDA/spack#401, we are going to define a variant +mysql for ewok-env that by default is false. We will provide instructions for users to set that variant to true and install an external mysql server (same as they do now) if needed. JCSDA will also maintain external mysql installations on the two systems that need it, with the necessary site config/documentation.

Additional context
This issue is related to #705

[climbfuji]

Closed via #993