Bootstrap command

[xen0l]

This PR adds bootstrap command, which allows to download session-manager-plugin and install it, so it doesn't require sudo. Fixes #15.

[codecov]

Codecov Report

Merging #18 into master will decrease coverage by 2.06%.
The diff coverage is 76.76%.

@@            Coverage Diff             @@
##           master      #18      +/-   ##
==========================================
- Coverage   88.23%   86.17%   -2.07%     
==========================================
  Files           8        9       +1     
  Lines         374      470      +96     
==========================================
+ Hits          330      405      +75     
- Misses         44       65      +21

Impacted Files	Coverage Δ
aws_gate/exceptions.py	100% <100%> (ø)	⬆️
aws_gate/cli.py	50.64% <28.57%> (-3.9%)	⬇️
aws_gate/bootstrap.py	84.33% <84.33%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 6dcbd80...6d86a39. Read the comment docs.

[alanjds]

My 2 cents: If you want to bootstrap the Linux plugin (as we made internally on GetNinjas), you can use https://github.com/getninjas/unix_ar

Our internal CLI is something alike this:

SSM_PLUGIN_PATH = {
    'darwin': {
        'download': 'https://s3.amazonaws.com/session-manager-downloads/plugin/latest/mac/sessionmanager-bundle.zip',
        'bundlepath': 'sessionmanager-bundle/bin/session-manager-plugin',
    },
    'linux': {  # yapf: nolint
        'download': 'https://s3.amazonaws.com/session-manager-downloads/plugin/latest/ubuntu_64bit/session-manager-plugin.deb',
        'bundlepath': 'usr/local/sessionmanagerplugin/bin/session-manager-plugin',
    }
}

def _extract_from_deb(debfile: 'filelike', targetfile: str) -> 'filelike':
    """Extract `targetfile` from debfile"""
    import unix_ar
    import tarfile
    logger.debug('Extracting from debfile: %s', targetfile)
    ar_file = unix_ar.open(debfile)
    tarball = ar_file.open('data.tar.gz/')  # default interest location on .deb files
    tar_file = tarfile.open(fileobj=tarball)  # handles gz decompression internally
    return tar_file.extractfile(targetfile)

def _extract_from_zip(zipball: 'filelike', targetfile: str) -> 'filelike':
    """Extract `targetfile` from zipball"""
    logger.debug('Extracting from zipball: %s', targetfile)
    import zipfile
    return zipfile.ZipFile(zipball).open(targetfile)

(...)

# download = io.BytesIO()
final_binary = open('session-manager-plugin', 'wb')
if platform == 'darwin':
    filelike = _extract_from_zip(download, SSM_PLUGIN_PATH[platform]['bundlepath'])
elif platform == 'linux':
    filelike = _extract_from_deb(download, SSM_PLUGIN_PATH[platform]['bundlepath'])

final_binary.write(filelike.read())
final_binary.flush()
os.chmod(final_binary.name, stat.S_IREAD | stat.S_IEXEC)  # chmod u+rw